The only way GTA Online would work is if people would co-operate, and that's not likely gonna happen any time soon. Since that's not likely gonna happen, we could expand the gangs from San Andreas quite a bit, so you can pick a gang to join. This way you need to defend your territory against the other gangs, so your gang needs to co-operate to stand a chance. And don't worry, even if you do work together you still get to blow up pretty much everything you want. Even more if one player drives a tank and the other one controls the turret. It's just a thought.
This again rules out all those who don't want to fight all the time, so perhaps we can have alternate things to do? Maybe you get play a civilian without weapons (to prevent a civilian player from blowing up everything) or a cop? Ambulance driver? Halicopter pilot? It has potential, but it's risky. And risky means "We might not get much money from this", which again means it won't happen. It's simply not safe enough. Sad but true.