Yay, great to see some of you guys liking my ideas
trade route: hard to make a realistic implementation.
the cities along the roads connecting 2 civilizations to eachother would have high trade commerce, is it ok? but how. well, we build road to every city. so which city would be more important for trade? in late game, nearly all plots in the landmass have road.
I think of the following model: You can stablish trade routes between cities only within a fixed radious of movement of a caravan, say, a unit with 4 moves. Depending on the tech development (compass, corporations, etc), civilization traits, buildings (marketplace, airport, etc) communications (roads) and natural communications (rivers, sea, etc), said "traderoute radious" can be expanded. Of course, things like closed borders and wars and civics reduce this ratio. You can stablish trade routes in the same way that you build roads in civ revolutions, you choose the citie (or cities) you want it to go to, and the further it is, the higher is the cost. Once the route is stablished, the benefits apply following these factors: time (the more ancient is the route, the more benefitial it is, for it has been consolidated), number of cities involved (the more direct and less intermediaries there are, the bigger are the benefits), distance (the further you stablish it, the more benefits you obtain) and interdependance (if the destination city lacks a resource the source city has or viceversa, the bonuses are multiplied). There you have it, a comprensible, realistic traderoute model.
immigration cannot be handled.
I think that not only it can be handled, but that it is, in fact, being handled in this new civ 5. I readed on a Spanish magazine that friendly civ could not only cross your frontiers, but also settle on them O_o
naval forces should be more useful yes. but i don't like naval forces anyway. why? i don't like maps with many landmasses. they suck in ways of resources.
It can be solved trought a more efficient resource distribution system.
Attack bonus against surrounded units: this is cool but in which directions should they be surrounded? all6?
I think of something like this: you get bonuses if you are attacking a unit surrounded by more of your units, but it also gets nullified if surrounded by friendly units. Example:
Unit surrounded by 3 friendly units and 3 hostile units: No bonus (they nullify each other).
Unit surrounded by 3 foes and 1 friendly unit: -50% defense (the enemy gets a +25% attack for each surrounding unit that it is not nullified).
differenciate culture into different aspects and Warfare evolving from one era to another: those would exploit the game a lot.
It is not as hard as it looks, it is just a game designing philosophy that hasn't been exploited yet. I made a mod for civ 3 (back when you didn't need to know how to program in order to change minor things) that addressed the evolution of warfare, even if it was just in small dettail: you did not get units with a bombardement radious bigger than one tile till the arrival of the gunpowder, calvary were the only ancient units able to move more than one square and tanks were made the first unit able to attack multiple times, thus enabling the blitzkrieg.
The differenciated culture aspects is not something impossible, I think of something like this:
Language:
Expanded trought: Trade routes and demography
Benefits: Cities with your same language have lowered manteinance costs. Incidentally, if some civ is able to expand their language into your borders that will cause you manteinance problems too, due to the increased bureaucracy needed in order to adminstrate the city into several languages
Religion:
Expanded trought: Same method as civ 4
Benefits: Civic bonuses, line of vision
Lifestyle:
Expanded trought: Happiness buildings, technology (once your civ is the first to research a technology, the adoption of said technology is considered to be the adoption of your lifestyle)
Benefits: Happiness penalty for entering in war against you, happiness bonus for being allied / supporting you in a war, easier assimilation of inmigrants
Arts:
Expanded trought: Theaters, cathedrals, great people
Benefits: Your luxury and food products have a bigger effect in another civs, making them more valuable to trade with
Once a foreign city have adopted all these 4 factors, that isyour art, religion, lifestyle and language, yep, you can claim it as rightfully yours. Also, that could lead to a very interesting specialization in culture aspects depending on the civilization: Americans would excell at spreading their lifestyle, Arabs at spreading their religion, the French at spreading their art and Spanish at spreading their language. It would be quite awesome, me thinks.