What I'm really wondering is how far you plan to go with everyone being playable.
By that I mean
Genjii, PC-98's unnamed fairies/midbosses, general fairies, Rika's tanks, yin-yang orb, Lily Black, Meimu and Mitori and other fan characters, Renko and Maribel, ZUN, etc.
Lily Black is a must for me if Lily White is playable. She was even included in Touhou Soccer!
And, Three Mischevious Faries and Kasen will need to be playable too now I guess!
There -is- in fact a limitation to what I can do. The game simply breaks past a certain point and I can't do a thing about it.
A few of those will be appearing as midbosses.
Several of them I don't even count as characters (The Yin-Yang orb its own character? Seriously?).
With that said....
Rika does use her tanks, a number of fan characters will be showing up (Though limited to bonus bosses), Renko and Maribel
are present and playable, and Lily Black will indeed be appearing, although maybe not how you expect.
The fairies I had already planned for, but despite the I Had Never Possibly Expected This happening and them being about to get spellcards, no. They're non-combatants and are staying that way. They play a fairly minor, but vital role, within the game, which I won't talk about just yet.
I'm not even going to attempt to plan for Ibaraki Kasen right now. It'd be pointless. But depending on how long development takes, she may get an appearance. I will say that, if Rinnosuke's inclusion didn't make it obvious (Along with what I just said above) I am not ignoring the expanded-works cast. Hell, even Akyuu's present (Playing Gamesaver, again)
...ZUN being playable? Stop giving me ideas. I might actually consider that one. We'll have to see how big Touhou 13's cast is though, as I've got a couple other people who are sitting at the "I have x slots left before the game breaks, reserved for 13 cast members, but I've got backup plans" situation right now.
assuming of course we ever get 13