Touhou itself is a doujin series, not in the sense of being a derivative work (though it has its influences) but rather as made by an independent developer as a hobby, and the creator ZUN has from the start been very open to the involvement of other creators to iterate on Touhou. The series identity is inextricably linked with the Doujin Spirit.
Still, you raise a good point: making the enormous assumption Reimu is in, how likely is Nintendo to involve doujin circles in the first place? I've seen some people try to detract from Reimu's chances saying ZUN disproves of Nintendo's stance on doujin, when in point of fact 1) Nintendo is only hash about derivative works of their own IPs, 2) Nintendo is more than happy to include Touhou fangames on switch, and 3) Nintendo even included Touhou music in Jam with the Band on 3ds (Japan only game). So, working with ZUN is on the table, doujin games aren't bad to Nintendo on principle. But would Nintendo trust news of their secret dlc fighter to several groups of amateurs? idk, it's worth considering.