If they're going as far as to give a hint in writing, chances are that Steven Universe character might be pretty close on the horizon. Banana Guard was heavily teased on socials before his reveal so maybe we'll be seeing this one come to fruition sooner than later.
I wouldn't put a lot of stock on it being a villain per se, mostly because of the limited options. It's also a bit difficult to classify villains in Steven Universe, just given the nature of the show. Maybe Jasper is the closest thing to a feasible Steven Universe "villain" I can think of, otherwise someone like Spinel would feel a bit odd played up as a bad guy. The movie goes through great lengths to empathize with her and promptly redeem her, so I can't say I'd feel too keen on her squadding up with The Joker and Jason Voorhees. Jasper would feel a bit odd too, but she is characteristically an extremely loyal and misguided soldier.
Reasonably it should probably be Pearl or Amethyst, with my preference skewing slightly toward Pearl. I can see them going in a wackier direction if they feel especially inspired by or personally attached to someone like Spinel or one of the secondary gems like Peridot / Lapis. Banana Guard, however easy it was to make him aside, kind of blows the lid off of any reasonable "hierarchy" of characters a series needs to follow. As long as you got a couple of the most important guys in there, I guess. Breaking this up before the OG Crystal Gems are complete doesn't make a ton of sense but maybe they'll value that variety.