Game-play wise, Android 21 turned out to be perhaps my best character in FighterZ, definitely one of my mains, so as long as in-game they're fun I suppose I don't really mind it, though admittedly I probably wouldn't advocate for it myself. I can see Android 21 coming back in other ways (I believe she somewhat appears in Kakarot after all) but I don't see them returning down that well after dedicating FighterZ's story mode towards the Androids. If they do continue down this path, I hope it's not all still tied to Gero, accepting 21 herself was a bit of a stretch, feels lazy to just keep saying "Gero had others still" after all these years.