I have some ideas for a city building game. The main insight for the ideas is that these mega cities that we like to build are a modern phenomenon, they are highly dependent on fundings from different organisms (federal, UN, etc). They also depend on a lot of knowledge of city management (traffic engineering, civil engineering, urban economy, etc). And of course, since we play these games to build megacities, the economical side is very important.
So some ideas are:
1- you can borrow money from different places. You can try to get money for this or that project from the UN, but then you'll have to obey their feminist, human rights, agenda. Get it? there are different sources of money but they all demand something, so where you get money from influences how your city develops, and it also influences your international politics. So you can create global cities out of a village of fishermen by borrowing money.
2- City-states only, preferably islands, a la cingapore. This for greater freedom and feeling of participation in a global economy and politics.
3- Universities are important, that's where you get your staff from. So you could set your policy towards universities, and this would influence what kind of secretary you could get, and also influence what kind of service you can hire (like if you don't invest too much in having good civil engineering courses, then you'll have to hire civil engineering companies from elsewhere to build your more advanced projects).
4- let them community create buildings, so that cities can have a wider variety of them
5- make it so that players can judge and evaluate aspects of your city that the computer cannot, stuff that players usually like to obsess about in city building games, like city beauty, city skyline, taming the terrain, etc.