We don't know how much characters they are going to add, but my guess would be 12 to bring the roster to a nice round 60. My line-up would be (other than Shou): Mamizou, Nue (I vote for some sort of random elements to make her as weird as possible), Kokoro (interactions with masks like you said), Toyosatomimi and Futo (to have our Taoist team), Ichirin (with some form of interaction with Unzan), Koishi, Yamame (would be like Hina, just for ailments instead of debuffs), Akyuu, Tewi (since she's an earth rabbit, she'll be completly different from Reisen, and we can get a Eientei full group), and 1 more.
I don't see them adding Letty (too similar to Cirno), the Prismriver sisters (unless they put all three in one, since they are pretty much all the exact same), Shizuka (too similar to Minoriko), Medicine (would be the same role as Wriggle, just not as a tank), anyone from PC-98 (we haven't gotten any from the previous game or any hints in this one, sorry Mima and Shiki fans) or anyone from the 14th game (the game is too recent, maybe we could get Seija, cause she was really popular). The three fairies of light would have to all be in if they decide to add them, it wouldn't make sense to only have 1, and they all have different abilities and spellcards, not like the Prismrivers. I'm still on the fence about Hatate, she has potential, but it could be easy to see her as filling the same role as Aya, and thus, have no use other than being a reskin of another character.
Also, with them adding new skills, I want a way to get Keine into Hakutaku form, that would be a fun gimmick, or anything that would be stance/form changes for characters
3. Two additional spells: A Self-buff that allows Remi to restore health equal to a portion of the damage that she deals for a certain number of turns, with said buff's duration increasing with leveling the spell up
I don't think she needs more spells, but how about a Vampirism skill instead? Heals everytime she deals damage or something like that. A skill for both her and her sister, would kinda make sense for them, since you know, they are vampires after all
About Aya's free turn spell:
It doesn't really give characters two free turns. Its just everyone interprets it that way because ideally, you'd switch your nuker in so that she manages to get her turn just before Aya's, she unleashes her nuke, Aya gives her another turn, she nukes again.
Also, just reminding everyone that the only benefit to leveling up her spell is when the character gets to act. If you keep the spell at level 1, the character would have her time bar set to 10000, which probably means she'll get her turn after other characters who charged their time bar normally. Leveling up Aya's spell to 2 would set the receiver's time bar to 20000, which guarantees that the recipient always act immediately after Aya.
Not only that, but some skills that activate when the characters gets a turn sometimes have the condition that their ATB bar needs to be higher than 10000 to activate (First Aid from the Healer subclass for example), so it does benefit from giving it 1 more level, but more leveling it higher than that doesn't have much point.
In other news, I'm in vacation! No school this summer, so my schedule should look like this:
1. Work
2. Study japanese
3. Finally finish translating this goddamn game
So look forward to it!