I think a PW could work but it needs someone in charge to prevent it from growing too quickly.  SWPW pretty quickly had more and more threads.  It's a good thing there's the interest but when it explodes that quick it immediately gets unwieldy.

Superheroes might work, IMO, if you kept them at a street level.  Maybe have people RP as villains or cops or rivals.  Somewhat like Arena where you set folks up with opposing forces.  If I'm Batman someone else is Bane, if I'm Daredevil someone else is Kingpin.  But it would rely on me and that other writer having a good rapport.