Three questions

Would it work, what platforms would it be on and Would you play it

*SMACK*

umm...there is....

There is, and of course barely anyone plays it. I imagine you we're asking about an MMO? No, it would have horrible character customization options in comparison to current MMOs and would be very stale(What Professions/Classes would their be?). The only thing it has going for it is the open city which would not be enough to make people fork over 140$ annually.

I mean a full blown MMO

for classes you could have Hitman, enforcer etc skills like marksman ship, brawling, driving etc.

It would only attract kids, the possible classes are their, but the main audience will say fuck that... Leveling driving skills? It isn't a racer and never will be considered one. Every class boils down to a gangster who fucks people up with guns or blunt objects: neither which is enough to differentiate a class. The closest they could do is a GTA SA type skills system and have it as the multiplayer options for the next GTA, but it wouldn't be a true MMO and no one will pay for it.

Be realistic:

What Classes could they have(not that they would work/be played)
-Gangster
-Gunsmith
-Bookie
-Mob Leader(imagine this would be something along the lines of a rank over a class

Can you think of others which wouldn't be some sort of ripoff of the 3 I listed?

How would weapons work? Their would be near no difference in weapons if it is left as an open shooter, as I cannot see GTA imploring RPG style combat.

It simply should be kept out of an MMO status IMO.