Provided the dev team gets around to adding another bloom form my guess is Flowering Peach. Peach has her bloom form already (sans art under normal circumstances) so why not add the bloom form for her cousin/sister/whatever?

As for Naee, right now I've been keeping her in slice form so I can take Nasturtium or Gerbera from level 1 to 80. 5* Knights are all I have left in terms of bloomable units so it's either one of those two or someone like Lily Rubellum (one of the few non-event 5* I have all equipment slots unlocked on) or yukata Mei Plumblossom (+crit rate, +attack power, +solar burst damage).