Touhou is basically the codifier for the bullet hell genre and Reimu is the face of the series. It's a HUGE indie series that has been going for at least a decade and has massive conventions in Japan and a pretty solid US fanbase. I'm not gonna say it
created a genre, but without Touhou there'd be no Ikaruga, for example.
Reimu is a big deal, but she's also a meme character because of how popular Touhou is on 4chan. Most people, ZUN included, would likely want to see her stay an indie character. That said, Touhou music is popular especially in pretty much every Konami rhythm game (Bemani) such as Taiko no Tatsujin and Pop'n'Music. It's very unlikely she'd ever be in Smash because of how the fans and ZUN feel that her being licensed to something that big would be going against the spirit of the character.
That said, with the projectiles, awesome moveset she's had in fighting games, and spectacular music and stage she could bring (seriously, you don't get more iconic than
UN Owen Was Her) she could be a great choice. Just, yknow, a totally impossible one. Reimu is to 4chan what Sans or Reckless Safety Notice Man is here, except maybe even more impossible.