I've mentioned elsewhere that if it had become a series, the most natural crossover in the world would have been with Here's Lucy, which ran from '68-'74 (AE aired in '68, and would've been a show by '69-'70.)
Crossing a mundane sitcom over with a sci-fi show seems unlikely, but then,
I Love Lucy did that George Reeves Superman crossover episode in a way that left it deliberately ambiguous whether "Superman" was George Reeves being addressed by the name of his TV character, or the actual Superman.
I'd think a
Mannix or
Mission: Impossible crossover would've been more likely, though.
Although the series name-drops Star Trek in the episode "Lucy's Replacement", shows have done that before without people caring (i.e. mentioned a show as a show, then later featured characters from it as if they share a universe).
Like how characters in
Batman watched
The Green Hornet on TV and characters on
The Green Hornet watched
Batman on TV, and then Batman and Robin met the Green Hornet and Kato in person.
As I've mentioned, though, I think it's entirely possible that an A:E series would have avoided any mentions of Trek or would've soft-rebooted itself to be out of continuity with the Trek backdoor pilot episode. So it wouldn't have been an issue in that case.
I'd agree with The Avengers but not Sapphire and Steel. That is a show that's truly out there. Nothing else like it IMO.
Yeah, which is why I was iffy about suggesting it. Still, it takes place almost entirely on contemporary Earth, and doesn't generally involve aliens other than the main characters, although it does tend to involve threats involving time in various weird ways. So as poor an analogy as it is, it's at least a bit closer than
Doctor Who, which just goes to show what a poor analogy
Doctor Who would have been, despite what people generally assume.
How about we split the difference between the various elevator pitches suggested herein?
Assignment: Earth would've been like
The Avengers if it had starred Klaatu and Jo Grant.