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.
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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.
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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...