So the topic of John Byrne’s mid-’80s reboot of the Superman line came up on Bluesky, where, among other topics, the new villains Byrne introduced was discussed. By and large Byrne took the probably-editorially-mandated approach of retooling some of the old Superman adversaries into something a little more modern-feeling, but between making Mr. Mxyzptlk unpleasant and making Metallo unpleasant, some new creations entered the mix.
…Aaaaand…dropped out of the mix, as for the most part, the new opponents had little to no staying power, or were used only sporadically until mostly just fading away. Like, say, Skyhook, or Dreadnought and Psi-Phon, whatever that one robot mummy thing was called, or even, God help us, Sleez (who apparently appeared about three or so times, which is four more than I expected). Even Bloodsport, who got a little play when a version of the character was used in a Suicide Squad movie, is mostly underutilized.
The one villain Byrne introduced who actually kinda/sorta got over, is Silver Banshee, probably more because of her striking appearance than anything else. Other creators took a liking to her and started developing her presence, and she also made the jump to other media (remember when she was on Smallville? She certainly was!). She was even showing up in the Superman comics relatively recently,
Which is a long way of saying that Byrne seemed to have more luck doing memorable stories with the established characters here than he did creating brand new villains for Superman to fight. But, really, that’s okay…superhero comics are littered with one-shot wonders who show up to fight the hero for an issue, gets sent down the river to do a nickel in the gray bar hotel, and are never heard from again. Byrne cooking up one villain (maaaaaybe two, if you want to count Bloodsport) that manage to make the grade out of however many he attempted isn’t a bad ratio.. I mean, it’s no Lee/Kirby cranking out legendary character after legendary character, but who could ever match those guys.
And that brings me to >Fantastic Four, as I’m sure you were expecting, given the title of this post an’ all. Byrne’s overall goal to his run on the Fantastic Four was that all-too-familiar phrase of “back to basics.” Byrne stripped the title back down to the Stan ‘n’ Jack days, focusing mainly on their creations as adversaries for the early issues, building on their foundation rather than much of what gone on in issues prior to Byrne’s takeover. (One exception was bringing back Terrax the Tamer, a cocreation with Marv Wolfman from Byrne’s previous run on the title, and who had his own measure of staying power).
Which isn’t to say that Byrne didn’t make some attempts at introducing new antagonists to the title. In #237, be brought in Spinerette:

…an interesting-looking lady with the power to induce vertigo-like effects:

…I mean, she got a “super-villain” name, she had an interesting look, but far as I can tell, only appeared this one time. Misunderstandings were resolved, she left the planet with her people, and that was that.
In #239, we get some critters that have been pestering a smal Arizona town:

…only it turns out they’re pals with a young girl that had been helping the FF throughout the issue:

There’s a lot goin’ on in this issue that I’m glossing over, but, even though Reed Richards his own self implies that these beings could appear again, far as I can tell, they never do.
In #266, one of the few issues during Byrne’s tenure that wasn’t mostly drawn by him (Kerry Gammill doing the honors, with Byrne pencilling a couple pages at the beginning and end of the issue, but inking throughout), we got Karisma:

…a villian with mesmerizing powers, shown here putting the whammy on the Thing:

…and she apparently turns up again fighting Captain America at some point, but she’s only had a couple of appearances altogether.
And then there’s the big one (literally). Terminus! I seem to recall Byrne stating somewhere that the intention was to create a Fantastic Four villain that would stand with the other classic FF bad guys, and, well, what we got was a riff on Galactus:

In Amazing Heroes #39, Byrne is quoted as saying
“The whole thrust behind Terminus […] is to introduce a cosmic bad guy who is a cosmic bad guy. Terminus is an extremely powerful menace, but he is not as powerful as Galactus, nor does he have Galactus’s nobility or important role to play in the workings of the universe. […] He’s not beyond good and evil [as Galactus is said to be]. He’s evil and he likes it. I don’t want a character with even a hint of nobility this time. He’s scuzzy. Every time he comes back, he’s going to be nastier. He’s basically out to have a good time, which he defines looting, pillaging, raping, murdering. He’s a kind of cosmic Attila the Hun.”
So, a giant bad guy who destroys planets but, unlike Galactus, is kind of an asshole about it. He’s sort of the answer to “oh, you think Galactus is a ‘bad guy,’ here’s what an actual Bad Guy Galactus would be like.” Terminus even has his own little companion who helps him track down worlds:

…and in contrast to whenever Galactus was defeated and driven away from the Earth, Terminus’s defeat results in him becoming trapped inside the Earth:

Interestingly, in an Amazing Heroes interview some years later, Byrne says
“…I had created Terminus for Marvel, and he was originally created to be a Darkseid-type character, although he evolved into something different.”
Of the FF villains Byrne created, Terminus seems to be the one closest to having the goods, though I believe Byrne only used him the once. He later turned up in an Avengers story by Roger Stern, John Buscema, and Tom Palmer, then was the focus of a crossover among a handful of 1990 annuals. He (or his armor) have made a handful of appearances over the years, making him at the very least a minor villanous player in the Marvel universe, if not the major recurring threat that Byrne had planned.
When I started reading Fantastic Four comics, I think even then I could feel the difference between an established villain and a new creation, why one had staying power and why the other did not. I enjoying reading the Spinnerette story, but I wasn’t left with a feeling of “I can’t wait to see her again,” or “this is the beginning of a long history with this character.” She was a short-term puzzle to be solved for this one story, with no real reason to return to her.
Terminus at least has some apparent depth to him, despite his one note purpose as “cosmic thug.” He’d clearly been around a bit, he had taken a helper from a previous world, his defeat left the promise of a return. This felt like a character with some legs, even if it would be under other creators, at least for a time.
Whereas, for example, Diablo, or the Puppet Master, two villains that turned up very eary in Byrne’s run, felt like guys who were absolutely part of the Marvel universe (because, well, they were) with understandable motivations and histories, and possibilities for further stories. These guys weren’t created as one-offs, they could be plot machines for many stories to come.
Not to say that Byrne’s new villains, aside from Terminus, couldn’t have been. Who knows what those little demon critters from #239 could have evolved into. Or what allegorical purpose Karisma could have served? But they were ultimately minor villains who existed to serve that one story and nobody felt inspired to dig them up again (or at leaset to dig them up too often). Again, that’s fine…not every character can be the Joker or Doctor Doom. And Byrne was hardly the only creator to contibute a new villain to the FF cause that made his or her one stand and was never seen again. Lee and Kirby are hard acts to follow, after all.