I vote we keep it simple, really.
You want a character that can merge two universes together? Mister Mxyzptlk (DC) aka Impossible Man (Marvel). A Fifth Dimensional being able to manipulate reality as will and canonically can hop between the DC and Marvel universes on a whim. And would probably do it out of amusement or to win a bet.

I didn't mean making amalgamation characters. Just that the worlds would be a mashup.
On topic, just how much of Marvel and DC's canon would we be allowed to use?

Deft, okay, that's cool. I'm jotting down a few ideas at the moment. Once I have enough, I'll post them here.
And Kafka, that idea is actually pretty freakin' awesome. I'd feel like a total thief if I did it though :/ lol