Speaking of Mima and the other PC98 characters, for this release the biggest candidate to come back wouldn't be Mima - it'd have to be Gengetsu and Mugetsu.
In addition to the obvious fact that the Dream World will presumably play a large part in 15.5's plot, and the fact that they're two characters who come pre-grouped together, they seem to be the perfect thematic fit for the game. They could exist either as the perpetrators of the incident and final bosses, or (like Yuuka in PoFV or Alice in PCB) bystanders who were roped in or joined the incident of their own accord. Even bringing back both of them isn't necessary, as ZUN could easily group Gengetsu by herself alongside someone like Doremy or Hecatia. Mugetsu could get in this way too, but she's far less likely to get in without her sister.
The only other circumstantial help that this idea has is the quote "The Strongest Two Have Joined Forces!," since ZUN once called Gengetsu "the most terrible, most fearful boss" in some music comments back when LLS was released - though this helps the idea that Gengetsu could be grouped with someone like Hecatia most, rather than Mugetsu coming back too. As far as evidence goes it's not very strong at all, but it's better than most PC-98 characters.
I'm still skeptical of the prospects of any PC-98 character coming back, though, much less in a game made by Tasofro.