That would be the most fitting thing if only it was possible. But alas she would only appear in the Spring in any given time. Anyways, like I've said before until some things start coming together on Toady's side, I haven't considered putting the Touhou charaters in yet since I want them to be different.
But hey as soon as creatures of the night come into play and assuming they are editable.... I should have an chance to throw in some actual Touhous.
When/if you put them in (actual characters, not yukkuri), will they be at full power, at hiso tensoku power, or at on of the shooters' powers?
Or will the absurdly strong ones never appear at all?
When I do put the charaters in, it'll likely when Toady finally make some head way with the magic system (which is a few years down the road). Because my main goal will be to use what I know about the charaters, and spellcards form my experience of playing the games and apply them to the actual game.
Though to be fair, I would probably have to simplify them because I could not possibly imagine DF being able to process alot of magical actions occuring at once.
But then again we don't know what magics going to imply all together so it may be wishful thinking on my end.
But here are something I'll put together explain some things I have in mind assuming magic will be customizable and workable once it's release.
Teleportation: this is important for yukkuri yukari as well as yukari herself to have this. It may not be crossing a gap specificly, but it would likely fit along the same feild. See this yukkuri randomly gap her way into your fort would be amusing.
Controlled blinking: More or less the only way I can think of to capture Sakuya's time control. Because as expected since the game runs in real time, we can't exactly freeze time so the second best thing would be that.

Useage of elements: Very important for nearly every chacter, especially Patchouli. To my view, having it so she could use spells like Agni Shrine, and Emarald Megalith as well as Royal Flare would be a plus, also I could see water magic being used to flood out your fortress.

Wall brakeing magic and lasers: Important for Marisa. The day we can make lasers and brake down walls will be the day I create a MArisa that'll fire the master spark at your fort. Imagine the lives it could obliterate!
Summon: Important for Ichirin and Alice, need a way to make it possible for them to summon assisit in battle. And hopefully down to specific summons. Otherwise symbiotic relationships will be important.
Symbotic relationships and war pets: This is another case of wanting specific things to be made avaliable. This would be important especially for Satori. As for reading minds, that just requires in an insane evasion rate (that is already possible)
Invisibility: This is already achivable, to some degree. But what I would like would be a charater to become invisible again. This is important for Koishi! Fading from the subconsious and killing unsucpeting Suikas would be a big plus.
Buffing: Important for Byakyuren as well as Byakuren yukkuri. Since we know her magics mainly about buffing more than anything else. But I would like to include some way to make those simlar features found in some of her spellcards too.
Powerlevels: Don't truely follow those all together. Though to be fair, I base the current size of creatures of Suika Fortress on a difficulty overall. This will however change in the future once Toady impliments size features. Because then for instance.... the Suikas will have to "climb humans" to litterally to stab to hurt them on certain areas (since they are suppose to be really tiny replicas to begin with.) If the Touhou characters themselves can be made more of a challenge from magic alone then they may stay roughly the size of a human. Othewise, well, we'll see what the future has in store...
There's many other things I want to consider but I will say for the time being, I do have some actual Touhou canidates in mind...
With creature of the night being an important adventure mode feature. This will allow me to begin a testing ground for certain TOuhou charaters.
For example:
Some victims can end up drained as subvillains or slaves in the same way a bandit leader has subordinates
Does anyone think of certain youkais that can do this? And of course there's also Remillia.
Killing them might ultimately require burying or burning a body, etc.
Since the youkais are truely incapable of truely dying, maybe there will be a wake to make them absolutely unkillable but at least make thier bodies disappear and reappear in an entirely differnt location!
Can be cursed by night creatures when you put them down
Does that not sounds like something that could happen if you attempt to kill Hina?
Diseases/plagues could be included as indirect torments, as well as famine once site resources can be assailed
This of course sounds like a Yamane thing.
So as you can see based on some of the ideas Today has been putting together I may have a way to introduce Touhous into this. Just not alot for now. They'll mainly serve as a testing ground for future charaters.
The canidates I have in mind are: Rumia, Mystia, Yamane, Hina, Wriggle and maybe Remilia
Though to be honest, I rather save Remilia for incase Toady decides to make something like a dark overlord that rules in a mansion of some sort. Then I would have something new planned for that. ^^ Imagine a mansion full of armed maids and Remilia being like the dark overlord ruling over them and if I can mixes others in, then Sakuya, PAtchouli and Meiling can be worked into the mix.

Like I said before alot of this stuff is in long term planning, but at least there's a good chance I could include some Touhou charater eventually once the night creatures are put in. I just hope these night creatures are "pyshical creatures" and not something like a entity like a god or something, otherwise this plan would go to waste.