Feel free to cross universes with wild abandon. For example, one team-up I'd like to see is Doctor Doom and the villain he must surely have inspired, Darth Vader. Actually, I'd like to see them fight.
But after they fight, I'd like to see them team up against Earth and Tattooine, with Vader attacking the former and Doom the latter.
How about you?
As you know, I'm a big fan of the Gorilla Grodd/Lex Luthor team-up/rivalry in Justice League. I'd love to see Mandarin and Doc Doom develop the same chemistry.