The absence of Star Trek wouldn't cause much of a dent in the sci-fi world. There's a 900+ year old alien from a planet called Gallifrey travelling in a police box to fill that gap...
Aliens in other sci-fi series will be more alien...or at least, less humanoid. Star Trek was the big promoter of humans with funny ears and/or foreheads. Compare to the aliens in Dr. Who or Star Wars, which, while generally not extremely alien, aren't as ridiculously convergent.
Given, that the female Aliens on TOS hardly ever looked the slightest bit alien i don't see how it would have made much of a difference. Sheridan & Delenn are the Pioneers there.Would we also see an end to interspecies lovin' subplots, or was that already well-enough established by Boroughs-style "space princesses"?
Slightly off-topic, but it's a question i've been meaning to raise and this thread is as good as any place: Could the German Raumpatrouille series ever have managed to take off?