Obviously a lot of people want a multiverse thing that doesn't die off so that's a good place to start pondering. A fun thing we could do is a team based thing where everyone can fight for a team to see what team gets the most points, although it never ends or has some insane end goal so we don't hit it too fast and than have nothing to do (or just have a sequel to reset things back to square one). The battles don't have to lock you into a certain place forever and lose interest, so you can join a bunch of battles as much as you'd like in the name of your team. They could be random battles, just agreed beforehand to be a part of this scoring system to keep rivalries going. You could also apply to fight for any team for that battle, not stuck with a single team, although it would make sense if you fought for a certain team only with certain characters. There would be a continuity but could be all over the place in terms of timeline so some battles could be fought between cavemen vs. dinosaurs while others would be modern, for example. This is also an excuse where you can use the same character in multiple battles at once because they don't have to be taking place at the same exact time, so they can logically be all healed up and everything.

On an unrelated note, I haven't done a battle with dice before but I suppose it could be fun.