Well, there's castmates over at outer_divide on offer :3 The game is more on the slow end, but it fits all your other criteria. There's a Tony, a Pepper, a Thor, a Sif, and a Tales of Asgard-flavor Loki :3

The game itself is log-based, though there is a network, with easy AC (five comments a month) and an over-arching plot that is at turns sci fi, political, and mystery. Right now we have an alternate-timelines plot going which is a lot of fun so far :) Plus, there's dinosaurs. Everybody likes dinosaurs >.>