Great contribution.
This is a very open ended question. Do you have sets you plan to incorporate? Or are you building from scratch?
In MOC's that I've seen in the past, obviously there are houses but to me there are 3 levels of City MOC's.
Setup with some buildings
Setup including buildings, roads, foliage, streetlights, all the extra stuff.
Then all of that with a train or monorail incorporated.
Those are just my ideas of different levels of complexity.
With more info from you, we can give more precise answers.