Why the hell would anyone want even more returning characters?
Best thing about these games is getting to know the characters and slowly piece together how each fit in the grand scheme of things. Even when they are returning characters, it works much better when we have absolutely no idea they are returning characters.
We already know two slots are meant for Sigma and Phi, so each time I read people hoping for the artwork to be A or B, I wonder if they'd want this game to be something like "Hey guys!!! Here we are again, huh? Bad luck... let's go figure out these puzzles!".
(We really know nothing about Diana, so she might as well be a new character)
We can have as many returning characters as you want as long as they are NOT participants, and appear only in the later half of the game. Please let this game spend some time on a whole new cast.