I don't know if it would be a good idea, i'm sure it'd be fun, but for some reason I just think it would take away from the original GTA theme a bit.
Another thing is, it would be very strange playing the actual storyline while other people across the world are causing complete mayhem in the city and stuff.
If they did have online play I think you should only be allowed to play online after you've beaten the story at least once... so therefore you get the real theme of GTA how it should normally be and then you get to go crazy online