Keep in mind ZUN also said Sanae wouldn't be playable in a game because she's a very strong character. He can change his mind at will to be quite honest.
yada yada spellcard
Pretty much this. I don't really believe that being "overpowered" should be considered as an exclusion criteria for any character to be in any game *cough*Mokou*cough* ESPECIALLY if the premises of the game this time is that the enemy is TOO STRONG you actually need to team up to beat her down.
There are already plenty of OP characters who can do just fine in the games, even in the shooter ones. How is the issue of being overpowered tackled in these ? Through various character related motives : just a test, playing around, not feeling motivated, you name it... in this game for example, we should get a "took 2 of you to beat me" approach toward the supposedly OP new character.
Point is, fitting an OP character into a game really shouldn't be a problem at all : making it coherent would be the challenge toward creativity. Which ain't that hard.
Anyway, there'll be one guaranteed OP character : that'd be the villain in this game's plot : now the issue of being overpowered comes in the question of how to not only make the character balanced (gameplay wise) while STILL giving the message that she is pretty strong (cuz Touhou lore gotta be respected, y'know). And I say there might just be one way to do that :
See the ULiL interview in Strange CoOW and how some things were in ULiL ? It kinda gives the impression that there are a few ideas that Tasofro didn't get to implement in ULiL, like have Reimu have a hopeless boss battle against Sumireko's shadow for example ; and some other stuff they actually got to put in the PS4 one, such as battling EX Mokou or EX Sumireko in arcade overdrive : these are pretty OK ways of handling the issue. Just a challenge towards creativity.
It'd be pretty cool if Tasofro got to put those ideas in this game. Hopefully they will