So my 2 cents.... If we are all going to do sections of one large city, then in addition to a general layout and how each section connects to the ones around it, there should be a general type of city predetermined. For example, is this a Large medieval city in a fantasy setting built by mixed races, or a High Elven enclave, or is it a current Earth world city like London, is it at it's peak or is it post-apocalyptic.
It would look silly if we all did our thing and we ended up with a Dwarven Garrison sitting next to a Space launch facility