Bridges: sometimes you have 2 bodies of land separated by 1 tile of coast. i think that some form of travel across this coast should be possible. 3 possibilities are--
a worker would build a "bridge base" on each of the land tiles that the bridge would be connected to, and then a work boat would create the actual bridge. the bases are so that the bridge knows where to connect.
work boats would have a "build bridge" icon, and when clicked, you would have to select the endpoints of the bridge.
when the "build bridge" icon was clicked on a work boat, all land tiles bordering the bridge tile would be connected.
the connected tiles would need to have roads on them. these would be modern bridges, requiring modern technologies. the bridges would be big so that boats could also pass under them.
another possibility with these bridges is to have a bridge with only one end connected to land. the other end would just be out in the sea. units would be able to go out on them. if an enemy ship was on a tile with a "pier," land units would be able to use the pier as a fighting platform and attack the ship. bridges would be able to be pillaged like any other improvement. if a land unit pillaged it, the bridge wouldn't be destroyed until that unit left, otherwise they would fall into the ocean. this could be used as a trap: pillage the bridge with a relatively weak unit, and leave it there. your enemy doesn't know that you pillaged it, so he will attack and kill you. your unit will be gone, so the bridge will blow up, dumping all unit(s) on the bridge into the sea where they will drown. the force of the explosion would also kill any sea units on the tile.
buying and selling land: when your neighbor' borders are pushing against yours, you want some way to decide who gets what other than constructing cultural buildings. am i right? there needs to be a way to trade tiles; to say "i'll give you these 3 tiles for those 2 and 1000 gold." traded tiles need either to:
have some of the culture of the civ receiving them, even if just 1%.
or
be bordering the receiving civ's current borders.
some system of deciding the value of different types of tiles (terrain, features, bonuses, improvements, distance from cities, etc).
advanced "explore" actions: when you tell your units to "explore" during a war, they just wander aimlessly, not always attacking, not always avoiding being attacked.
---there needs to be an action that causes the unit to walk towards areas where enemy units were recently spotted, or are often spotted, and if enemies are found, KILL THEM!
---another action needs to exist to tell them to go to tiles where resources/improvements are known to be and pillage those tiles.
---also, units need to be able to be commanded to map out enemy territory, going where you have least recently been to bring you updates on what's there. the command shall include "ad if you see any enemy units, end the turn by hiding yourselves in a defensive location (hills/forest/jungle/fort)."
---a "go fortify yourself in the nearest safe city/tile" is useful. a "safe city/tile" is defined as a city/tile belonging to a civ with whom you have open borders and your opponent does not. this does NOT include your cities, as your enemy can get there. if your enemy opens borders with the civ who's land you are hiding in, the unit will automatically relocate.
---"go home" is self explanatory.
ghost units: the last time you passed a city, you probably saw units in it, and you want to remember what they were so that you can guess now what you'll need to bring to conquer it. you probably can't see what's in that city now, can you! "ghost units" would show "ghosts," or semi-transparent images, of what was there last time; you see an axeman, but you also see a farm through it. when you hover over the city, it shows the list of units on the tile as usual, bit in a different font and/or size and/or color to show that it is a "ghost list." there would also be an option to "ghost" a certain tile, a tool that would be activated and function in the same way as ping, and would store what was on the tile in ghost form each time you passed it.
teleportation: you've all wanted at some point to rush a unit somewhere fast, but all the local airports are used up for the turn or non existent. want a solution? a future technology would be "teleportation," which would allow you to create "teleporter" units. these units could move themselves and one other unit (or just itself) to any other tile on the map each turn. the target tile would have to be accessible normally (you have open borders with the civ who's land it is, it isn't impassable terrain). these units would be extremely costly to build, and valuable once you have them. however, they wouldn't be able to defend themselves, as they would-like workers and settlers-have no strength and are captured if an enemy unit enters the unit's tile. a 50% chance of escaping by teleporting to your capital would be had by teleporters. since that is not losing a battle, it is winning, and therefore the teleporter gets XPs. the promotions would be different from all other units, giving it abilities to see what was around the target tile before teleporting, giving it a better escape chance, being able to carry more units, or teleport twice; since these are valuable promotions, they would cost much more XPs than promotions for other units. maybe treat promotions like research, each time it earns a promotion, you put more points towards the promotion of your choice.
nukes that kill units and buildings more powerfully: we all have had the frustration of using a nuke, only to find out that instead of destroying and killing, it only caused some unhealthiness per turn. these death sentences would destroy what it hhit and what was around it. when targeting a city, you get a more detailed city map, and choose which part to target. that area of the city is destroyed. when targeting a tile other than a city, any and all improvements, resources and units on that tile and all adjacent tiles are destroyed, turning the tiles into desert/tundra with fallout.
resource planting: you have sheep, but you want more sheep. a special "farmer" unit should be able to spread plant/animal resources by taking seeds/baby animals from your existing resources (improvements req) and planting/raising them on other tiles. different types of resources would require different technologies. horses can be spread with horseback riding, health plants with agriculture, health animals with animal husbandry, and luxury plants with calendar. farmers would be available with agriculture, the wheel, AND pottery. maybe a limit of 1 farmer per era per civ at a time? seeds/young livestock would be tradable through diplo with paper.
advanced "open borders, etc": have you ever opened borders with someone to scout out their territory, but wished that they wouldn't park their tanks in your yard? advanced open borders would let you say which types of each player's units were allowed in your territory. you could also exchange gold per turn (gpt) for rights ("i'll let your civillian, recon, melee and archery units in if you let in my recon and pay me 3gpt"). when bargaining in diplo, you can also ask the other player to change his border status with other players ("i'll let your armored units in if you keep A's gunpowder out of your territory and let B's siege in").
Superheroes: a unit that grows stronger as time goes on! this would be a special unit (not nessicarily called a superhero) that would become stronger as you gto more advanced units. its strength would be calculated with a mathematical formula, here is a suggestion (all strengths don't include promotions): 2 * the strength of the strongest unit you have + the strength of the strongest unit you can build + 1/2 the strength of the strongest unit any discovered opponent can build + 1/4 the strength of the strongest unit you have ever seen. it would be an air unit that instead of being based in a city/fort, could move around on each turn on any tile. it could also go over water too, but if it was gone from land more than 2 turns it would die. movement would most likely be double the movement of the unit with the greatest movement that you can build (roads and (if applicable) railroads included). each civ would start with ONE, and would NOT be able to build more, even if theirs died. i'm not sure what to picture them as, a dragon rider, a spartan warrior, a muscular guy in a cape, some geek with a tech-suit that can do all sorts of stuff (like jetpack, shoot lasers, etc). maybe appearance would change with the era. if the unit has space for an emblem/logo (on the cape, shield, color of dragon) than those would be the civ's colors/emblem that you would find on their flag. they could engage in combat like a land/sea unit (enter a tile with an enemy and dish out the pain) and like an air unit (see a grid, drop bombs on one tile). they would apply for all the normal promotions (bonuses in different situations) but would also have special ones (more movement, more strength, extra visibility, can stay at sea longer). they would have no penalty for amphibious assault. heroes could bombard city defenses and attack any category of unit. a possibility is intercepting rockets in a space race victory-when someone's rocket takes off, you attack it, damaging it so that it takes longer to reach its destination; if this is possible, then a good idea is to have your hero guard your ship during takeoff. these units would be f***ing bad-a$$ powerful, but having only one of them means that you'll have to watch out during war and keep your hero healthy and with lots of movement, or the enemy's hero will circle around and pwn you from behind while yours is on the other side of the earth, leaving you at their mercy. this really is a SUPERhero with SUPERpowers!!!!!