My personal fave is one completely forgotten by everyone. pajama-mode yuuka ='(.. everyone puts her in that plaid outfit, whyyy. qq.
If that's the case, why not make an item that gives Yuuka that look as an alternate costume? Of course, given the nature of the game and its cut scenes, it would be kind of hard to imagine how a cutscene with obtaining said clothes would go...
I find that despite mima having the most vocal fanbase amonst the pc98 girls, shinki always seems to be the one that the game devs favor.
Shinki makes for very good boss material, whether it's a final boss, a bonus boss, or whatever. I think that it's easier in terms of imagination for them to think of attacks for a creation goddess is all.
I think the PC-98 character that has the biggest chance of coming is Mima, but other characters would also be really nice to see (Elly and Shinki maybe)
Mima is kind of more in line as boss/player character material and only if they bother to put her in. Some games have done so, like Devil of Decline, but still...
Elly, on the other hand, would fit in due to Yuuka, so that'll work out. Working out a moveset for her would be the only issue, but if they can invent one for Kasen(Lot2 predates ULiL), then they can certainly make one for Elly.
You know who I would be surprised to see, but would not question the appearance of?