First on the guys we believe are "benched." Eraqus and Xehanort seem super dead now. I can't really picture them returning except as a brief appearance as force ghosts to give some encouragement or something. Seeing them again in the form of a prequel I'm cool with. I'd love the see their pasts and even if such a prequel lead into future stuff, Eraqus and Xehanort aren't actually back.
Ansem, Xemnas, Vanitas, and Dark Riku/Replica Riku. Now these guys I'm suspicious of their status. In the Ultimania, those four are simply listed as "eliminated", unlike the other Org. XIII members, who are given slightly more explanation. I mean there is more said in Dark Riku's status except it's not specifically on what happens to him, and Terranort's lists him as his heart being eliminated but that hurts my head as he's Xehanort's heart so if his heart is eliminated instead of going back to the past then that screws up everything big time.
But with these four dudes, nothing is said on what happens. Not that they went back to the past. Not the head-hurting "their past hearts blew up. Wait, wouldn't that..?" It's like it's being deliberately vague on what happened to them, and Vanitas' death scene gave me vibes he'll return.
Dark Riku and Replica Riku, I dunno. Dark Riku should have just returned to the past as he's Replica Riku from the past (that's what the Ultimania says), except Xion was also listed as from the past (specifically saying from when she first joined Org. XIII) though also says that she regained her memories from her original heart within Sora. Meaning the Xion we first see is a new heart and her and original Xion merge or whatever. If Xion can be listed as from the past except she really wasn't, then it could be Dark Riku was another new copy as well? Since Replica Riku also needs to be mentioned when talking about DR here, he did the glowing light thing that Eraqus and Xehanort were doing at the end so maybe he's meant to be super dead now too, I dunno. The story being all "clones are people too!" and Replica Riku is just "There's already a Riku. lol Bye." kinda burns my brownies.
For everyone else, I don't want any characters "benched" exactly, but Yen Sid's role in the series needs to be toned down heavily. His main role of Mr. Exposition in KH3 was completely redundant, wasting everyone's time by having him just reiterate what's already known, and just not really doing much at all. I want him going back to just having brief appearances, like in KH2, and not try to shove him in at every plot point.
What KH really needs I think, is shorter games to better focus on individual groups and plotlines, instead of trying to make each title a "full" 20-30 hour game by stretching things out with bad decisions.