So... planning the various orgin events I was thinking about nemeses... villains that have their origins linked to the heroes in some way.
This can be overdone. As is a recent trend in super hero movies of late.
But I think a nemesis, if done correctly can be valuable. Properly done, the nemesis can:
1. Provide a recurring villain for the hero, the first suspect.
2. Work in the background if needed, doing inexplicable things which only later make sense (and so can provide a deeper element to the world)
3. Have a built in motive that doesn't involve self-aggrandizing (although they certainly can be that as well).
4. Be a sort of 'rock' for the character to cling to when doubting their role. For example: Batman may see the crime on the streets growing, a villain escape or be released from custody on a technicality. But when the Joker shows up and has some sinister plot, it is difficult for the Batman to just shrug his shoulders. He is part of the reason the Joker is out there doing his evil. And so he can't just leave it alone. (Substitute Superman and Lex Luther, X-men and Magneto, Spiderman and the Various Goblin(s), etc..)
In a group game, one might think a singular nemesis for all of them (like Magneto, or Dr. Doom) could be the way to go. But, there is a value to an individual nemesis which seems more important. A group of individual nemesi(nemesises?) could on occasion team up, but they should probably be special to the character most of the time. I think.
So problem. If everyone in the group has a special nemesis, it seems a little lame. Unless there are lots of solo adventures, the point of a nemesis might be lost in the rotation.
Different kinds of nemesis.
1. The revenger - the hero is somehow responsible, in the villain's eye, for some serious loss (injury, spouse, humiliation, bankruptcy etc.) This could also be an NPC hero.
2. The former partner or teammate. The reformed villain might have old allies show up as enemies of the group. They might be especially nasty to the traitor, or try to lure him back.
3. Parent/Sibling/Child/Creator - Similar to the above case. Ambivalence is a good source.
4. Rival - Scientific, Economic, Acrobatic, etc.. The hero and the villain share some apptitude, the villain cannot allow the hero to be better than her. So she does what she can to humiliate the hero and prove him second best.
5. The 'Ex' - always a volatile combination super powers and the love/hate relationship. This can also work with 'Unrequited love', 'Super Fan' a la The Incredibles, or Ex-friends after an affair or the like.
Should these be introduced at the origin, or can they be worked into the game later. That is a tough question. If they are integral to the character becoming the hero, they might need to be brought in early.