Yes, Mon Mothma is a very minor character. Especially before the Andor show, which arguably gives her too much screentime anyway. General audiences wouldn’t freak out, they would say “who is she supposed to be?” You would have to develop her for a little while before you make it a big twist.
I’m not too familiar with the Yuuzhan Vong but I think that’s the kind of direction you have to go if you don’t want to repeat the Sith and the Empire again. Lucas’s idea of underworld criminal elements filling in the power vacuum left by the Empire is also good (though I’m iffy on it having Darth Maul.)
Being that this is Star Wars, I don’t believe there are ‘minor’ characters anymore. That’s why every scene in the Mandalorian has some kind of callback be it an alien, a piece of hardware etc. We even had the cantina guys repeat their '77 dialogue in Rogue One!
It would be so easy to generate a nostalgic connection to Mothma in ep 7. One early appearance in an RoTJ-esque outfit, with Leia saying “it’s so good to see you my friend - we’ve missed you terribly” would bring a rousing cheer and establish a ‘between films’ connection that would fuel the Ben Solo thread. Maybe he attended university in the Bothan system and stayed with the Mothmas?
The criminal underworld thing is plausible of course, but as much as we hate to admit it we do want the Sith and Empire again! Of course we do. Casual audiences definitely do. Again, the Mothma idea at least gives credence to the notion of a reconstituted status quo rather than the obvious ‘uh, the Empire grew back’ thing of Zhan’s novels, the Dark Horse comics, and the sequels. Again, it leaves the OT victory entirely intact whilst presenting the sinister idea that one of the lovely characters in the Rebel briefing room was actually a baddy with a future axe to grind…
Yes that kind of thing sucks, it’s something people both fans and outsiders complain about all the time. Seemingly every single character in the Mos Eisley cantina was given their own fleshed out backstory and adventures. Sometimes minor characters are just minor characters. The fondness some people have for minor characters out of universe does not mean they suddenly had more screentime retroactively added to the movies. Easter eggs are not the same thing as main characters.
“Uh, the Empire grew back” is not at all what happened in the Zahn books or the comics. The Empire is described as being on the back foot, fracturing into groups of warlords, not what it once was but not completely destroyed either. It makes perfect sense. The Empire starts winning in the Thrawn books because of Thrawn. He’s a singular, once in a generation military genius who lets them consistently win battles for the first time in a while, and he makes good use of technology like cloning, and various aliens. They also have help from a powerful dark jedi.
That makes so much more sense than Mon Mothma being a secret bad guy and rebuilding the Empire in secret, unless you basically made her into a Thrawn figure. That’s much more “Uh, the Empire grew back.”