CureMoonlight said
This is actually a pretty darn solid idea. That way those who want to play OC's can play OC's and the others can pick and choose from the characters and canon antagonists without that big of a hassle, at least in my opinion.
They can do that anyway though. If the character you want to play is dead/otherwise unavailable by the end of their series, you can probably just say those events didn't happen/have yet to happen in this timeline. Although really, I'd say characters should just generally not be how they are at the end of their series, kind of awkward trying to make villains a threat when all the heroes have their best power-ups available from the start, ehe. That's just my opinion though.