Here’s possibly a few more to add to the “might have been list”. This has not been verified:
Joel may have mentioned in an 1989 interview they wanted to do “Mars Needs Women” (this would have been on KTMA)
Before committing to using “This Island Earth” for the “MST 3K: The Movie” experiment, there may have been interest in trying to riff “World Without End” (which they did for their first live show in 1992. Here’s a link to a thread I have up on that: https://forums.mst3k.com/t/mst-alive-1992/16525
Mike mentioned they were looking at “Day of the Triffids” during a pre-Season 8 TV Guide article. He also mentioned in the Masked Reviewer interview they tried to get “Baffled” at some point.
“From Hell It Came” was supposedly passed over for “Deathstalker” due to rights problems.
Kevin Murphy mentioned “The Land Unknown” in an online chat during the Sci-Fi era as a movie they were gonna do back in season 8, but didn’t.