First of all , I will try to explain myself but my English is not very well. I am so sorry for my grammar mistakes but you will get the idea.

My idea is a Space City its like titan but much much bigger.almost as much as the size of the planet.or bigger ! It will have 4 tech levels.you will be able to choose. 3 diffrent option at each level. those options are Economic city ,military city, cultural city Each modifications provides unique features. and every level city will grow

Example : if you chose 4 military city modification, you will have a very powerful space ship that will have capabilty to destroy big fleets, titans and planets too. (like death star) and it will have hyper space engine (but its very slow)

Or, if you chose 4 economic city modification, it will have their own gravity zone thus you will able to build orbit buildings..

 there is a lot of combinations can be made.

Thanks for reading my idea. I hope somebody take this idea serious and we play soase with huge cities !