So what are you doing about quests? Or achievements, something to do other than just walk around? I think thats something you should think about. As when i made my first multiplayer game kind of like this, coding up quests gave me code i needed for the regular game itself, so doing that quite early could help you alot.
Nearing the end of the first development of the game, if its going to be the kind of style like GTA (Free roam do whatever you want in a map kind of thing) you may want to think about players being able to create their own maps, like car races or deathmatches.
