Once, in a joke-fic, I had Willow Rosenberg pair up (despite her preferences) with Little Joe Cartwright. The readers were left to wonder whose bad romance luck would kill who first, as Buffy got a report of a demon buffalo herd heading in their direction.
Crossover romance is one thing. I often wonder about crossover logic. For example, in the DCU or MU, wherein aliens are a known fact of life, would Mulder and Scully investigate government links to known unfriendly aliens, like the Skrulls or Dominators? In a combined DCU/MU world, who would be the pre-eminent uber-team? Would the JLA and Avengers, who spend time in the public opinion crapper, trade places when one is on the outs? Would anti-mutant activity be diminished, or made greater by the doubling of the super population? Would the arrival of the Tenctonese (from Alien Nation) in the 80's prepare Earth better for first contact in 2063 or would the Vulcans prove the tipping point in species' tension? Would Doctor Strange be one day called upon to be Headmaster of the American analog to Hogwart's? Could Willow be his apprentice, or could one of the Halliwells, sparking a feud between the Elders and the Vishanti?
I have to stop, now.