While it makes sense with AoCF as context, it would be weird for that to be a Touhou character's gimmick; perhaps it should be one of a few? On top of that, smash has always found it hard to balance transforming characters: too often, one is clearly better/best. Their removal from 4 may have been motivate by technical limitations, but that allowed Sakurai to make Zelda, Sheik, Samus, and ZSS more well rounded characters without extra weaknesses to encourage switching. Pokemon Trainer's return barely works in this regard, as it took major overhaul to the switching mechanic as a whole, and frankly is ingrained more deeply into the character to where it needed to work. And even then, people still argue that some of the pokemon are better than others. What I'm getting at is I believe Sakurai is predisposed to be wary of transforming characters for balance reasons, on top of the development strain of developing a two-in-one fighter on a dlc staff.
IMO, to make transforming an appealing enough mechanic you have to go beyond giving each character compatible strengths (itself a questionable balancing method) but also give switching itself some benefit. Making it essentially another airdodge is a start, but maybe you'd take a page from AoCF and give it healing properties as well.