[quote=Laughing Matter]
How about some sort of Crisis-type event? Something and/or someone is screwing with the fabric of the multiverse or whatever, and worlds are leaking into one another. This cosmic bullfuckery causes previously dead canon characters to come back to life, in case someone wants that sort of explanation. The OCs would have their own little world/universe while the canon universes would have their own, obviously. The OC universe has something to do with this crisis, which is why various characters are all crossing over there.If it's too OC-centric for you, you can simply re-tweak it if you want to use this idea.
[/quote]

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.