That reminded me that I had heard of a Star Trek/X-Men crossover (apparently what I was thinking of was this comic book). If they did a movie crossover, Patrick Stewart would get to negotiate between the Federation and the X-Men as both Captain Picard and Professor Xavier. That scene makes the whole project worth doing.
And from that point, we tried to connect the cardinal science-fiction and fantasy franchises to each other. Jim Broadbent connects Harry Potter and Narnia; Christopher Lee connects Star Wars to Lord of the Rings . . .
It took some brain-racking (and eventually some help from IMDb), but this is what we came up with:

Incidentally, several of these franchises to Kingdom of Heaven. I'll let you flesh out the connections in the comments.