The problem with crossovers between SF/fantasy universes, even aside from licensing issues, is that they can really only work as "imaginary stories." It's one thing to cross over two present-day series like, say, Cheers and St. Elsewhere, or Law & Order and Homicide, since they're set in essentially the same reality. 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. For instance, there's no way to do a legitimate, in-continuity crossover of Star Trek with anything where first contact with aliens was overtly made before 2063, such as Alien Nation or V. Nor could you cross over a universe like Star Trek or Babylon 5 that has faster-than-light travel with one like Firefly where it doesn't exist, or cross over ST's alien-rich galaxy with the humans-only galaxy of Asimov's Foundation universe or the 2004 Battlestar Galactica. And how do you cope with something like the original Galactica, where the cosmology was so ignorantly handled that ships travelling below the speed of light could travel across multiple galaxies in under one year? There's no way to even attempt to reconcile something that fanciful with any universe that pays even a moderate amount of attention to astronomy and physics.
So the only way to do it is by fudging the inconsistencies, ignoring whole huge swaths of worldbuilding and history in both universes -- which I consider cheating, since those worldbuilding details and backgrounds are fundamental parts of what make SF universes what they are. Such stories can be entertaining artifacts, amusing exercises, but I can't really take them as legitimate parts of either universe.
A crossover between SF universes could [only] really work in cases where the physics, history, and other aspects of worldbuilding are compatible. For instance, I choose to believe that Roddenberry's pilot The Questor Tapes takes place in the Trek universe, because there's nothing about it that fundamentally conflicts with anything ST has established.... But I don't count Genesis II/Planet Earth as part of Trek history because its version of WWIII and its aftermath is incompatible with the timing of those events in Trek (although I do consider it an alternate timeline where the Eugenics Wars became a full-fledged global nuclear war). I would've loved to incorporate Alien Nation (the TV series, not the mediocre movie) into Trek history, since they were thematically similar in the ideas and issues they explored, but there was just no way to reconcile them. SF universes that are compatible with one another are very rare, because the whole nature of SF entails creating whole universes with their own distinct rules and identity.
