Probably way too late to do anything like this, but I had a thought. The oldschool MMO thing doesn't fit the genre so much, wall-to-wall criminals in city zones stretches the suspension of disbelief a little--*but* I had a thought. There has been a recent game that has a system that *would* fit the genre. Warframe.
It's much more like the Super Hero genre to stick around in your hideout, or super group base, or possibly even a city zone for RP and markets and such, until you 'hear about a crime,' read: select a mission that it'll automatically load you into, you can go save the day from--much like how warframe has its mission and lobby system.
What do y'all think?