Star Trek and Alien
If a Star Trek movie involving Borg invasion or boarding a Borg ship was as scary as Alien, with the same uncertainty as to the fate of the principal cast, there would be no need for a ST-Alien crossover.
Star Trek and Stargate Atlantis, Farscape, etc.
Too many direct references to Star Trek in those shows. Farscape is so out there anyway that it's hard to cross over. The closest it gets - to my knowledge - is that hilarious Stargate-SG1 200th episode where they have to contribute ideas for a show based on their adventures and they pitch through the whole panel of science fiction shows.
How to work around a show established as fiction in another
A time traveller used their knowledge of the future as a basis for a so-called fictitious show. (Like it was done within Stargate about Stargate (Wormhole X-treme! ep.) - except without time travelling.)
Why they shouldn't produce crossovers
Crossovers must be fun in fanfics as they are in imagination. It's impossible to resist daydreaming one, especially with the help of the notion of alternate universes - and skipping universes also allows to skip in time. For example, wouldn't you like to bring nanites from another universe to cure the Drakh plague in B5 Crusade? Especially as this show was never finished.
But personally, whenever I watch a crossover on TV, especially at the beginning (Just caught the CSI-Without A Trace one tonight on French TV - an acceptable crossover), I get an uneasy feeling that they're expecting us to stretch our suspension of disbelief - to talk politely. I'll never forget how dumb it felt the first time I saw a crossover in an animated series as a child.
If a Star Trek movie involving Borg invasion or boarding a Borg ship was as scary as Alien, with the same uncertainty as to the fate of the principal cast, there would be no need for a ST-Alien crossover.
Star Trek and Stargate Atlantis, Farscape, etc.
Too many direct references to Star Trek in those shows. Farscape is so out there anyway that it's hard to cross over. The closest it gets - to my knowledge - is that hilarious Stargate-SG1 200th episode where they have to contribute ideas for a show based on their adventures and they pitch through the whole panel of science fiction shows.
How to work around a show established as fiction in another
A time traveller used their knowledge of the future as a basis for a so-called fictitious show. (Like it was done within Stargate about Stargate (Wormhole X-treme! ep.) - except without time travelling.)
Why they shouldn't produce crossovers
Crossovers must be fun in fanfics as they are in imagination. It's impossible to resist daydreaming one, especially with the help of the notion of alternate universes - and skipping universes also allows to skip in time. For example, wouldn't you like to bring nanites from another universe to cure the Drakh plague in B5 Crusade? Especially as this show was never finished.
But personally, whenever I watch a crossover on TV, especially at the beginning (Just caught the CSI-Without A Trace one tonight on French TV - an acceptable crossover), I get an uneasy feeling that they're expecting us to stretch our suspension of disbelief - to talk politely. I'll never forget how dumb it felt the first time I saw a crossover in an animated series as a child.