People were generally answering with who would be most likely and for that, Sakuya would be the most likely pick for obvious reasons. I intended the question more in the direction of who would have a interesting moveset or is a personal favorite.
Tough for me to answer the question, as I have a ton of character I adore.
Patchouli would be interesting with the potential for a complex gimmick sorrounding her different schools of elemental magic. Changing between the elements like Shulks monado, which in turn would switch up her special attacks and might even influence her normal attacks visually. We have a lack of pure magic characters and Patchy would be a good choice in that case.
Nue Houjou would also be cool, as I really want a spear user in Smash at some point. Given, she uses a trident, not a spear but eh... close enough. Her gimmick could be to make items in casual play unidentifiable. Not the most interesting ability, but a gimmick is a gimmick. Her short dress could be problematic for the cero rating, though.
But there are so many that I would love to see. Yukari, Tenshi, Remilia, Sakuya, Flandre, Eirin, Alice, Satori... and the list would keep on going and going.