Ayumi is a very weird character to try to think up a moveset for.
As much as I would enjoy her as a fighter, it's hard to picture her literally fighting. Seeing as
The Girl Who Stands Behind is likely the more popular of the two titles, you would probably see her primary appearance be based on that.
That would allow them to implement the ghost story in the final smash or something. But realistically I think she would make an excellent assist trophy.
Ayumi is a very dependable and resourceful individual, so her in an assisting role would just be a natural fit. However I know nothing about the third game, and I'd rather keep it that way in case it ends up getting remade as well, but she is the playable the one there and I am unaware if she has any physical confrontations in it. I do know that there is old concept art of her with a spear though lol
That said, I think this is a unique situation where the character probably better fits a secondary role more than a main one. I do really want to see things like music implemented, and while I think a stage may be a bit bland considering the iconic locations being just a school and a village, if they wanted to implement some of the rumors told in both games literally, they could have some pretty interesting hazards.
A side note, but it is a shame the only music represented in Smash was a snippit of the opening theme for
The Girl Who Stands Behind in Brawl's
Famicom Melody. It unfortunately didn't return in future titles because the Medley was rearranged.