Alright, pros, cons, and iffy for Reimu
Pros:
Touhou is very popular in Asia
Nintendo was at Reitaisai selling remixes in 2016
ZUN said that he's ok with Reimu in Smash
Accoding to one of ZUN's friend who helps him with legal stuff, Nintendo would "support Touhou in various way if the games, such as EoSD, were ported to the Switch", and this was said days before it was anounced that AoCF would eventualy get ported to the Switch
The copyright would be extremly easy to get for Sakurai/Nintendo
???:
Nintendo owns 15% of DeNa, a company who has been one of the sponsors for Touhou since 2018 iirc
Iffy:
One of the composers for Ultimate also does Touhou remixes
Touhou fangames are doing prety good on the japanese switch e-shop, but since those are fangames it's iffy
With Reimu being a shrine maiden and all, the religious stuff may go against her even if it pretty much did nothing to stop the Belmonts of Joker from getting in
Cons:
While Touhou is very popular in the east, it's not extremly popular outside of Asia (even if I personaly believe that we already got the 2 big hitters of the pass with Steve and Sephiroth)
Mainline Touhou has never been on Nintendo consoles, only AoCF got a port aounced, and said port hasn't hapened yet
The Touhou fangams on Switch don't get advertised by Nintendo, not even in indie worlds