Roleplay, definiatly. Roleplay is awesome in mods of older GTA games, and it will be amazing in IV:MP, because of the width ammount of skins, vehicles, etc.
The only problem is, we should have the ability to close down 2 islands or something, because you need a server with like 1000 people to get whole Liberity City populated, and that's impossible.
So perhaps a option to lock off the bridges for admins just like when you just start in Single Player?