If you're talking about making GTA 4 2D mod then that would be too much work, both you and I know that. It's nearly impossible to create this but none the less a cool idea.
Well your best option is to make 32 player suitable sized maps that represent parts of the city so yea. Even then it would take ages to make a complete if not partially completed map pack
Yep, I agree with Phenixtri, a part of Liberty City should be big enough for 32 players. Because Liberty City is huge. Even if you only take the airport, it should be sufficent for a whole 100×200 (or bigger?) map. But if you really WANT to make the whole work, go ahead, dude.
How do you want to realize the streets that don't have a nice 90° angle. These are kind of impossible in the current CS2D engine. Either you'd have to ignore them or you use awful tricks.
The cities (or parts of them) from GTA and GTA2 should be easier, because in that games, all streets have a perfect 90° angle.