This is just a brainstorm post that I am making up so I can refine and refer to them (and you could add feedback / ideas of your own pertaining to the subjects written within too).
There are a few problems I have with the game design right now. The first thing is I don't really like the global happiness mechanic. Sure, if I replace the word happiness with stability in my mind it makes it tolerable, but it seems too simplistic for my tastes. I really enjoyed the revolution mod introduced by the modding community in Civ 4, so here are some ideas to enhance the game to reintroduce the idea that might make the game more enjoyable for me.
Second, it seems that happiness and to an extent research does not scale well with the map. When playing a terra map there are few incentives to colonize the new world as it will slow down your social policy adoption, and hamper happiness. On the other hand, the larger the map, the easier it is to research as you can have more total population with the extra land. So on one hand larger maps are bad for civilizations focusing on culture, and the other they make the game progress to endgame more quickly.
Third, city states need some balance.
Basic premise: Making happiness a local variable for each city. A new global variable will be created called 'Stability' that will have a more important effect on your empire. The main things that will add stability to your empire are social policies, buildings and surplus happiness. The things that will reduce stability are unhappiness, number of cities, distance from government center and puppet / annexed cities.
A lot of what I am thinking right now will tie into social policies, so I will just start listing the changes I think would be interesting to explore and see what it does to the game. Some new buildings will also become available if you adopt certain social policies, giving more variety to your empire depending on specialization. Additions / changes are colored in red.
Tradition
By Adopting Tradition you get a +1
food in your capital (good for small empires), available from the Ancient Era.
Each new era reached provides an additional +1
food for the capital
Liberty
By Adopting Liberty the training of Settlers is increased by 50%, Cannot be active at the same time as Autocracy, available from the Ancient Era
Each new era you enter each non foreign city gains +1
Stability
went ahead and formatted all the other policies, but will add the bonuses they receive at a later date.
Honour
By Adopting Honour you get a 25% bonus to attacking barbarians and you get a notification if a barbarian camp is founded in your territory, available from the Ancient Era
Piety
Culture/Happiness focused, Unlocking Piety gives +2 happiness, Cannot be active at the same time as Rationalism, available from the Classical Era
City State Focused, Unlocking Patronage makes City State Influence degrade 25% slower, avalable from the the Mediaeval Era
Trade and naval focused, Unlocking Commerce boosts gold output of your capital by +25%, avalable from the the Mediaeval Era
Technology focused, Cannot be active at the same time as Piety, Immediately enter a 5-turn Golden Age, from the Renaissance Era
Upon adopting Freedom, Specialist Population in the cities produce half the amount of unhappiness, from the Renaissance Era
Order will increase the production rate of all buildings by 25%, available from the Industrial Era
Adopting Autocracy gives you a 33% reduction in the amount of Gold you pay for Army Maintenance, Can not be active at the same time as Liberty and Freedom, avalable from the Industrial Era
List of new buildings
... will add when I include them in policies
List of changed buildings
... might tweak library and a few others after I run through the policies
There are a few problems I have with the game design right now. The first thing is I don't really like the global happiness mechanic. Sure, if I replace the word happiness with stability in my mind it makes it tolerable, but it seems too simplistic for my tastes. I really enjoyed the revolution mod introduced by the modding community in Civ 4, so here are some ideas to enhance the game to reintroduce the idea that might make the game more enjoyable for me.
Second, it seems that happiness and to an extent research does not scale well with the map. When playing a terra map there are few incentives to colonize the new world as it will slow down your social policy adoption, and hamper happiness. On the other hand, the larger the map, the easier it is to research as you can have more total population with the extra land. So on one hand larger maps are bad for civilizations focusing on culture, and the other they make the game progress to endgame more quickly.
Third, city states need some balance.
- Maritime - primarily for use by small empires
Provide less food, but in return they will let your cities retain % amount of food on growth. The fewer cities you have, the more bonus food you receive.
- Cultural - primarily for use by large empires
Provide cities with culture directly instead of a global amount.
- Military - general purpose, slanted towards smaller empires
Provides a bonus for fighting in your territory and lowers the maintenance on units by a certain % (might scale with empire size to make them more attractive for smaller empires)
Basic premise: Making happiness a local variable for each city. A new global variable will be created called 'Stability' that will have a more important effect on your empire. The main things that will add stability to your empire are social policies, buildings and surplus happiness. The things that will reduce stability are unhappiness, number of cities, distance from government center and puppet / annexed cities.
A lot of what I am thinking right now will tie into social policies, so I will just start listing the changes I think would be interesting to explore and see what it does to the game. Some new buildings will also become available if you adopt certain social policies, giving more variety to your empire depending on specialization. Additions / changes are colored in red.
Tradition
By Adopting Tradition you get a +1

Each new era reached provides an additional +1

- Oligarchy - +33%
combat strength for military Units fighting inside the empire's borders.
Walls, Castles and Military Base provide +1Stability, +1
Culture
- Aristocracy - +33% production when building wonders
Each wonder built provides +3 Stability - Monarchy -
Gold cost of buying land reduced by 50%, requires Oligarchy
Garden +3Happiness, +2
Stability
- Landed Elite -33% faster growth of capital, requires Oligarchy, Aristocracy
Double production of Granary, Watermill, and Windmill, they are also maintenance free. - Legalism - Reduces unhappiness due to the population in the capital by 33%
Every citizen in the capital produces +2science
Liberty
By Adopting Liberty the training of Settlers is increased by 50%, Cannot be active at the same time as Autocracy, available from the Ancient Era
Each new era you enter each non foreign city gains +1

- Citizenship - Worker construction rate increased by +25%
Workers build improvements 25% faster - Collective Rule - Newly-founded cities start with 50% of the food necessary to gain their second citizen.
+2Stability per city
- Meritocracy - +1 happiness for each city you own connected to the capital, requires Citizenship
- Representation +1 culture in every city, requires Citizenship
- Republic - +1 Production in every city, requires Collective Rule
Double production speed of workshop
went ahead and formatted all the other policies, but will add the bonuses they receive at a later date.
Honour
By Adopting Honour you get a 25% bonus to attacking barbarians and you get a notification if a barbarian camp is founded in your territory, available from the Ancient Era
- Discipline - +15% combat for units that stand next to another unit
- Military Caste - Each city with a garrison reduces unhappiness by -1, requires Discipline
- Military Tradition - Units gain double experience from combat, requires Warrior Code
- Professional Army - Gold cost of upgrading Military Units reduced by 50%. requires Military Caste
- Warrior Code - Free Great General (appears outside your Capital)
Piety
Culture/Happiness focused, Unlocking Piety gives +2 happiness, Cannot be active at the same time as Rationalism, available from the Classical Era
- Free Religion - 2 Free Social Policies, Requires Reformation, Mandate from Heaven
- Mandate of Heaven - 50% of excess Happiness added each turn to the amount of Culture that may be spent on Policies.
- Organised Religion - Happiness required for Golden Age decreased by 25%
- Reformation - Immediately starts a 6 turn Golden Age
- Theocracy - 20% unhappiness from population in non-captured cities, requires Organized Religion
City State Focused, Unlocking Patronage makes City State Influence degrade 25% slower, avalable from the the Mediaeval Era
- Aesthetics - Minimum Influence level with all City-States is 20
- Cultural Diplomacy - Quantity of Resources gifted by City-States increased by 100%, Happiness from gifted Luxuries increased by 50%.
- Educated Elite - Allied City States sometimes gift you Great People
- Philanthropy - +25% more Influence gained from Gold gifts to City-States
- Scholasticism - Allied City-States provide a Science bonus equal to 33% of what they provide for themselves
Trade and naval focused, Unlocking Commerce boosts gold output of your capital by +25%, avalable from the the Mediaeval Era
- Mercantilism - Purchasing items in cities requires 25% less Gold
- Merchant Navy - +3 Production in all coastal Cities
- Naval Tradition - +1 Movement and +1 Sight for Naval combat units
- Protectionism - +1 Happiness from each Luxury Resource
- Trade Unions - Maintenance paid on Roads and Railroads reduced
by 20%
Technology focused, Cannot be active at the same time as Piety, Immediately enter a 5-turn Golden Age, from the Renaissance Era
- Free Thought - +2 Science from every Trading Post
- Humanism - +1 Happiness from every University
- Scientific Revolution - Gain 2 free Technologies
- Secularism - +2 Science from every Specialist
- Sovereignty - +15% Science while the empire is Happy
Upon adopting Freedom, Specialist Population in the cities produce half the amount of unhappiness, from the Renaissance Era
- Civil Society - Specialists consume half the amount of Food they would usually do.
- Constitution - Culture doubled in all Cities with a World Wonder
- Democracy - +50% Great People production rate
- Free Speech - -25% of Policy Culture costs
- Universal Suffrage - +33% City Combat Strength
Order will increase the production rate of all buildings by 25%, available from the Industrial Era
- Communism - +5 production per city
- Nationalism - +25% attack bonus when fighting in friendly
territory - Planned Economy - 50% reduction in unhappiness from the number of Cities you have
- Socialism - -10% Gold maintenance cost of Buildings
- United Front - Other players City-State Influence Points decrease
33% faster
Adopting Autocracy gives you a 33% reduction in the amount of Gold you pay for Army Maintenance, Can not be active at the same time as Liberty and Freedom, avalable from the Industrial Era
- Fascism - Quantity of Strategic Resources produced increases by 100%
- Militarism - Gold cost of purchasing units reduced by 33%.
- Police State - -50% Unhappiness in Occupied Cities
- Populism - Wounded military units inflict +25% more damage
- Total War - for 20 turns all units get a 33% attack bonus
List of new buildings
... will add when I include them in policies
List of changed buildings
... might tweak library and a few others after I run through the policies