The problem with crossovers between SF/fantasy universes, even aside from licensing issues, is that they can really only work as "imaginary stories."
stories basically imaginary?
But different science fiction universes tend to make different assumptions on a fundamental level: the way the laws of physics work, the way history unfolds, what alien species exist and on what planets, how the geography of the galaxy or universe is laid out, things like that.
Agreed. That's why I NEVER want to see a Star Trek/Star Wars crossover. I can only enjoy Star Wars as a strict fantasy; there isn't a shred of actual SCIENCE in it anywhere. Some argue the same is true of Star Trek, but I disagree. Some of the Star Trek episodes have been inspired by real science, and some of them have gone on to inspire future real science. And at least Star Trek never used the word "parsec" as a unit of time!
The only way I could even begin to make my Sliders/Xena crossover make sense was to have an AU where Alexander the Great didn't die young, but made it all the way across Asia and over to North America. And since the time setting of Xena skips around from biblical to current year with NO attempt to explain the discrepancy, I figured I didn't need to worry too much about the Xenaverse showing up in modern-day California instead of ancient Greece.
I didn't have the same problem at all with my Sliders/Handmaid's Tale story, as it's all AU in the near-future. For me the challenge is handling a double set of Sliders characters, plus getting the right "voices" for the Atwood-created characters.
I read a Star Trek/Dune crossover awhile back, where an older Federation starship (circa Captain Pike's era) ran afoul of an anomaly of some kind and ended up in the Dune universe, just prior to the events of the first of Frank Herbert's novels. None of the canon ST characters were in it, which was refreshing. What made this so much better than most Dune fanfic I've read was that the people of the Imperium were freaking out about the starship's computers (since computers are considered anathema in the Dune novels). And then the Guild noticed they no longer had a monopoly on FTL and they
freaked out. So right away the Star Trek characters had an awful lot of people upset with them just for existing, let alone before they'd had a chance to do anything.
So you can combine unlikely SF universes if you anticipate the problems and come up with a creative, in-universe solution instead of either ignoring it, using technobabble, or sneering that the readers "just don't understand" (Kevin J. Anderson's M.O. when he's called on his mistakes).