[suggestion] Thoughts on expansion
I don't like, that I can build a very large empire without any real penalty. To keep together a large empire always was difficult. Rebellions, seceders, lack of real influence in the distant parts of the empire.
The other problem with this, that it enable you to conquer everything before you reach the 2/3 of the techtree. So you can't fight with your best units against the other factions best. Or you can play on hard or above, which leads to the stream of armies on the AI's side.
I was speculating on realistic ballance, which doesn't throw away the actual mechanism.
I think the "prestige" could give us a good oportunity. Philosophically in the game the presige is your global authority. If you haven't got enough authority you cannot manage a great empire efficiently. So lets connect the size of the empire and the efficiency with the prestige.
My idea:
The number of prestige determines how many cities you can manage without penalty. The first two cities would be free, but after you need three (or less/more) prestige per city to avoid penalties. If you haven't got enough prestige you aren't able to settle (but you could build outposts). Or you could build one or two more cities above your prestige limits, but your empire would suffer. In this case you have to deal with some new challenge. The prestige buildings would be really worth building. And you can always conquer an enemy city, but that would cause additional penalties because of your insufficient authority.
The penalty would be a global unrest, and all of your cities would suffer if you don't care about that. So you have to build prestige buildings, solve it by spells . . . I think this could cause an 5 percent unrest per cities above your limit. So the captured cities could have 100 unrest if you expand too fast. Ok, you can raze the, but in this case you loose a good city. Maybe raising a city could cause 5 per cent unrest as well for five turn (decreasing by one percent each turn).
This mechanism would punish the large and help the small. This would lead to a more ballanced game. The AI could have less penalty on chalanging difficulty and could loose all penalty on hard and above.
The authority should be a new index. It would be the difference between your prestig, and the prestige needed to maintain your empire on maximum efficiency (which is determined by the number of cities). If it negative, you would get 5 percent penalty from -1 to -5, 10 percent penalty from -6 to -10 and so on. If it is positive, you are ok, and you can see how much other cities can you have without penalty.