I have several ideas for games.
One idea I had would be a city building game with some Civilization elements, but not done in the same way as CivCity: Rome.
One thing I didn't like about the historical city games that I played is that there seems to be no progression in technology. From the beginning you can build everything and the houses can develop to their full if you meet their demands. What I would like to do is introduce some sort of technology tree so that when you research the technologies, new buildings become available and your city can develop into a more advanced society. If you start at the beginning of history the people's houses would merely be wooden huts and as you advance, their homes will change as long as you also supply their needs. One example of how technologies could work is when someone is carrying a resource to a warehouse. In the beginning, the person is merely carrying it, but once you have the certain techs, the person will have a cart, making him faster, and you can build roads, making the route faster, and more like that.
Another related thing I thought of would be when you enter a new age, the overall style of the city will change. When you enter the classical age, your city will have a classical style, in the medieval age a medieval style, and so on.
The centre of the city would be the palace, and as your city grows you can build add ons, and in a new age have it redesigned if you wanted. The palace is where main decisions will be made. Certain buildings can't be built until the city has reached a certain size, like the rewards in the SimCity games.
One thing I didn't like in CivCity: Rome that didn't appear in other city building games is that people come to the city even if you don't have housing. I liked the other games where people only come to the city if you have the housing.
For how the people get their supplies like food I would have a system similar to that where instead of someone from the marketplace visiting the houses, the people from the houses go to the marketplace. The people from the marketplace would go to the granaries and warehouses for their resources, and the warehouses to the main industries and suppliers.
There could be a world level. Once your city has reached a certain size you can build settlers and send them out to found new cities. I think you'll have limited control over these cities otherwise it may end up being too complicated.
I haven't figured out how the military would work. Maybe something involving building certain buildings like an armoury where you can train someone from your city into a unit that you can use. Maybe they can be commanded in a real time strategy sort of way. Maybe there could be a mode where the city map is large enough for several cities and you can play that as a sort of multiplayer skirmish, except instead of just building a base, it's an actual city.
There could be a religion element in the game, but haven't though much on how it could work. I don't want it to be like Immortal Cities, which was quite confusing in trying to please your population's needs to worship certain gods. Maybe there could be competing gods if polytheistic and later other religions that you can build worship sites depending on what religions are practiced, and maybe you can send missionaries out to other cities. I'm not sure what else it could do, maybe another form of control where you can influence other cities.
I would like to have wonders, but I'm not sure how their effects would work. One at least would be a general improvement to the city's aesthetic I guess, but for others I'm not sure.
There are probably some more ideas that I could fit into the game, but I find it hard to explain things or know how things can work.