NOTE: This isn't a complete list of every idea I have, more so a rough outline of all of them. That's why, for example, there's not much information on changes to policies. Also not that this is the idea for vanilla. And that should be, tell me what you think!
CIVILIZATIONS (Default):
Social Policies (by era):
Ancient:
Tradition- Best for tall empires, this tree is geared towards a small number of big cities.
Liberty- Best for wide empires, this tree is geared towards rapid expansion and maintaining such a large number of cities.
Honor- Best for warmongers, this tree is geared towards having a stronger and cheaper military.
Piety- Best for empires looking for a religion, this tree is geared towards founding a religion and producing faith.
Classic:
Patronage- Best for diplomatic empires, this tree is geared towards more easily making city state allies and gaining more bonuses from them.
Aesthetics- Best for cultural empires, this tree is geared towards maximizing tourism and culture.
Medieval:
Commerce- Best for economic empires, this tree is geared towards producing and saving vast amounts of gold.
Zealotry- Best for evangelical empires, this tree is geared towards reaping extra benefits from your religion, especially your official religion.
Stewardship- Best for empires desiring health and greater benefits from the terrain, this tree is geared toward increasing the raw output of the land.
Renaissance:
Exploration- Best for naval empires, this tree is geared towards increasing gold, tourism and military power through the seas.
Rationalism- Best for scientific empires, this tree is geared towards increasing research.
Ideology (Freedom-Order-Autocracy)
Each ideology is viable for any victory condition.
New Mechanics:
Corporations
Available starting in the industrial era. Each coorporation is unlocked by researching a specific technology and founded by sacrificing an appropriate Great Person, who becomes a Representative. When founded, a civilization can be either Freelance or Government Funded. Freelance cooperations cost no money but also provide no money, while a Government Funded one costs gold to maintain, but also provides gold for cities it operates in, with the payout being larger in larger cities. If a company is freelance, a civilization can buy it and becomes it's new owner.
Cooperations are spread via representatives, which are provided by founding them and by training them in cities of yours in which the cooperation operates. They are like missionaries, though rather than remove inquisitors, Cooporations can be removed with a click on the cooperation menu and even outlawed. However, this produces unhappiness. Having a cooperation functioning in a different nation increases tourism to that country.
Health:
Unhealthy is caused by population and certain terrain types, as well as contact with unhealthier cities. If a city is unhealthy, it grows a bit slower. The more unhealthy, the greater the penalty. A very unhealthy city might even be the spawning point of a plague, which can deal notable damage to your growth, happiness and productivity, making it important to watch your health. Plagues can spread via trade routes and city connections, though a healthy city will be more resilient. Come late game the Hospital is unlocked, and health is unlikely to keep being an issue.
Energy:
Energy is basically electrical access. It is provided by building certain buildings and can be spread from generating cities with certain improvements. Energy provides access to various benefits and is needed to build certain buildings. Energy is provided by the following buildings:
Factory (requires coal, additional bonus of flat production)
Hydro Plant (requires river or coast, additional bonus of production on coast and river tiles)
Solar Plant (requires desert, additional bonus of gold on desert)
Nuclear Power Plant (requires uranium, additional bonus of flat production)
Wind Farm (an improvement which must be build on tiles without forest/jungle, provides gold as well)
A city must pick one of the 'plants' for its energy needs, or Wind Farms around it can provide energy as well. Once alternative energy such as Solar, Hydro, Nuclear or Wind is established, the unhealthiness caused by Factories is cancelled out.
City Distances:
Combining Civ4 and Civ5, both buildings and cities cost maintenance. Flat maintenance increases with distance from other cities, especially the capital. However, trade connections like roads or harbors, as well as other cities forming a network, reduces this, more so if those cities are connected to others. There is also unhappiness which arises from settling to far from your other cities as well. However, both gold and happiness are easier to come by, so such settlements are not implausible to found and settling conventionally is a bit easier. However, founding on a completely different continent separated by a large body of water makes these penalties greater.
When founding cities after reaching the Renaissance era, one can choose to have it be a Colony. Colonies cost much less maintenance and are less unhappy about their distance, but you cannot control them, like with puppets. Colonies can be 'annexed' at any point, making them function as normal cities. One thing to note is that Puppets no longer have the science penalty, though they do produce less culture.
Taxes:
Your empire has a 'tax slider', similar to the science or culture sliders from Civ4. Increasing it will provide more gold depending on the amount of population in your empire, but also increase unhappiness. Decreasing it will not reduce happiness, but gold as well. The scale does, for familiarity, go from 0% to 100%, but unhappiness increases exponentially as the slider goes up while gold only goes up linearly.
CHANGES:
New Buildings:
Cemetery: +15% Great People generation, available at Masonry.
Supermarket, +5 food, available at Refrigeration.
Airport, changed so it now establishes city connections and allows air trade routes to be established in/from this city.
Police Station/Courthouse, changed to reduce unhappiness from number of citizens by 15% and 25% respectively.
Intelligence Office, reduces enemy spy rate. Does not require a Constabulary, instead it requires the National Intelligence Center national wonder to be constructed. Once constructed, this cities potential (determined by things like science output, population, etc.) is added the empire Total Potential, and you get spies for every certain amount of Total Potential. By the late game, spies are everywhere.
Vivarium, +1 food on desert tiles, city must be on or adjacent to desert, available at Fertilizer. Borrowed from Beyond Earth, this building produces +1 food on desert tiles. This makes these locations more viable without Petra, and makes Petra's bonus towards the tiles even more potent.
New Units:
Cargo Plane, allows cities with Airports to establish air trade routes, which are more effective than Sea Trade routes and are most of what you're going to run come the late game.
Ranged Mounted tree, continuations of the Chariot Archer line. It was always odd how the Chariot Archer, Keshik and Camel Archer could gain these ranged promotions and then them become useless, so to change this and be more consistent throughout the game, there is now an upgrade path consisting of ranged mobile units.
As with Gatling guns and machine guns, the Ranger, Combat Jeep and Armored Assault Vehicle only have one range by default.
Fighters have been split into Biplane/Fighter/Jet Fighter/Modern Fighter.
Bombers have been split into WW1 Bomber/Bomber/Jet Bomber.
Stealth Bombers are built separately, and are some of the most powerful and expensive units around.
The Melee naval promotion tree now looks like this:
Galley (Ancient)-> Brigantine (Medieval)-> Caravel (Renaissance)-> Iron Clad (Industrial)-> Destroyer (Modern)
The ranged naval promotion tree now looks like this:
Trireme (Classical) -> Galleas (Medieval) -> Frigate (Renaissance) -> Dreadnought (Industrial) -> Battleship (Modern)
Combat Changes:
One Unit per tile is still mostly in effect: Each land tile can hold a civilian unit and a combat unit. However, any three combat units can combine at any point in the game. This increases strength and combines there effects, but not all the time (Archer+Spearman does not have a bonus on melee when ranged attacking, for example). A unit can enter and leave these squadrons in the same turn, so units can pass through one stack to enter another beyond it. Units with higher move paired with lower move units (for example, Chariot Archer+Swordsman) usually increases movement, but not the level of the more mobile unit on its own. The stats of units in the squadron are a mix of those in the stack, though their strength is notably more than one unit.
Combining multiples of the same unit on their own results in a stronger version of that unit, though your fire power/resilience is reduced compared to when they were spread out. A squad of three archers isn't quite as effective as three archers on separate tiles. This is mostly to make moving units easier or performing large scale assaults where you have to many units to use them all when separate. Combined units share experience gained and damage, though there higher power means that the damage is close to what it would have been if it was spread across them all after attacking one.
Cities are a bit weaker early on in the game, so a rush of warriors is a notably more viable strategy. However, come the late game they're notably tougher nuts to crack than in civ5, and building certain buildings actually gives them multiple ranged attacks.
There are now 'equipment promotions', which cannot be earned with experience. An example is Winter Clothes, which negates penalties on Tundra and Snow. These are unlocked by certain technologies (though some are granted passively by terrain before the promotion is normally available) and can be gotten on a unit either by increasing their production cost or spending gold once they've already been built.
Tiles
General tile yields have been increased, and the land is more fertile and potent on its own, more akin to civ4 than civ5. Hills can provide food and production by default, lakes provide 3 food, and tiles adjacent to a river produce +1 gold again.
Most units in tundra and snow will suffer penalties: Normally this is combat penalties, though units will also lose HP unless they possess the Winter Clothes promotion, and equipment promotion granted automatically to cities near tundra and snow. Snow Hills and flatlands now produce something, though are still weak.
Ice can no longer touch the mainland, opening up the map more for naval units.
Great Admirals can now establish a Marina on coastal tiles without resources. These provide +1 food, +3 production and +3 gold, as well as a 50% defense bonus to naval combat units stationed there. While a lighthouse boosts the food of the coastal tile the Marina is built on, it is otherwise not improved by things like God of the Sea or Seaports.
Forts and Citadels can be used to hold air units and naval units when on the coast. Forts can also be constructed in conjunction with an existent improvement and no longer remove forests and such, but cost maintenance. Citadels do not cost maintenance.
Expansion Costs:
Cities produce both unhappiness and cost gold. However, these resources are more available. Tiles tend to produce gold and cities produce +1 gold in addition to their default 2 food 1 production. You start with enough happiness to maintain two expansions even on deity, if they aren't spaced very far. Luxuries provide more happiness as well.
Local Happiness is a bit more important, and a city which is locally unhappy will grow and build pretty slowly, though as in Beyond Earth the severity is determined by just how unhappy: the penalty increasing exponential. Unhealthiness is completely city dependent, and also slows growth and generates unhappiness. Now, Global Happiness is still important though: An empire which is globally unhappy receives penalties to the local happiness of all cities and research will be slowed. One city of well kept individuals and three which are miserable and wallowing in disease will not progress unless those issues are addressed.
Religion and Espionage Changes:
Civs can now have a state religion. This is what affects your relation with other civs (rather than religious presence in your empire). Founding a religion does not make it your state religion.
State religions spread at an increased rate within your borders and affect your relations with other civs. In order to make a religion your state religion, you must have at least one citizen following it. Rest assured, that citizen will become many citizens rather quickly. However, having a state religion also produces unhappiness for those who follow other religions.
Espionage unlocks much earlier in the game and can do many more things. Not only can your spies sabotage construction and temporarily disable buildings, but they can also destroy documents (such as Defensive Pacts or Peace Treatise) and steal Great Works in addition to their normal information gathering.
Spies now become available in the classical era instead of the Renaissance, and by the end game many more spies become available. When they first become available, however, low potential means they're more for information and sabotage than tech stealing.
Trade:
Largely the same, though Caravans and Cargo Ships can reach farther than ever by using another city as an in-between point. Friendly and Allied city states can be used this way, as can civilizations you set up an Open Trade agreement with. These can allow more lucrative trade routes to be fulfilled, but unless the in between point is one of your cities than part of the gold and science goes to the in between city rather than to you, and for internal trade routes part of the food and hammers will go to the in between city instead of the city the trade route is actually to. However, resource diversity, unknown technology and other modifiers the in between city has will affect the trade route, potentially making up for the things they take. Cargo Planes cannot do this.
World Congress:
You can leave or not join the world congress. Some resolutions have been added, such as Free Trade, giving automatic Open Trade deals between all members and city states allied with them.
Diplomacy:
As in Beyond Earth, favors can be traded instead of an actual resource. The game keeps track of what these favors are for and they have their own tab, as with Other Players and Cities. So, a favor for '50 gold' will not be equivalent to a favor for 'declare war on X'. You are expected to fulfill these favors, but every time the favor is refused to be repaid, the diplomatic penalty becomes greater, more so the longer the favor has been sat on. This means asking thrice for the favor to be filled in three turns is a minor one, if any, while asking thrice in 100 turns is going to be taken far more seriously. This all depends on how big the favor is, of course.
One change is how asking civilizations to war is: You can ask them to target certain certain places or cities (they may drive up the price if it's difficult to set up their troops there) and even reserve ownership of the city once or if they take it, at a higher cost. In addition, when two civilizations work together to take a city, they can negotiate who gets it. The one who did more to take it will be given a naturally higher right to it, but enough compensation can give it to the less involved player as well.
Maps can be traded again, though technologies cannot.
One can also form a domination alliance with other civs. Depending on the size of the map, you will be able to join with a certain number of civs and take over the world with them. In addition, relations are a bit more personal: If Napoleon has no opinion on Elizabeth and you back stab Elizabeth, he often won't care. He'll be happy about it if he hates Elizabeth, or very angry if the two are allied. It's very possible to become close to warmonger or the like and then the two of you go on a rampage which causes everyone to hate you while the two of you are still friendly.
City States return and work similarly to Civ5. However, there are some differences. Mercantile City states provided a unique luxury and additional happiness, while Militaristic city states can gift unique units when allied. Based on this, Cultured City States give a special Great Work to their ally. Another change is that City States can now be razed, and if a cultured, militaristic or mercantile city state is captures their unique bonus can be taken from the city. This happens as with any great work for cultured, the luxury can be ported to be based in another city with mercantile, and the empire gains the secrets of the unique unit for militaristic.
-12, they go into civil war, and 4 low-level units rise around the capital. If they take the capital, then you lose all of the piety tree, and have to adopt rationalism
