I think it all depends on what the type of game you want to run…
If you want us to be stranger at first, and come together because of some crises, I think it has to be very local and very dangerous; something like a giant sea lizard attacking the city or a giant robot. That way we all have a reason to come to the same place (not like a bank robbery, which I don't think is going to cause a city wide alert, or we all 'just happen' to all be patrolling in the same area)..You could even start the game with all of us already fighting it, or just finishing up defeating it. Maybe whoever was controlling the monster got away… we have to band together to track them down.
Maybe someone is forming a new super-team, a corporation, a rich hero, the city itself. They put out notices and we all show up to join this team…
Hmm lots of ideas, but I have to know more about the game jasamcarl is going to run.