Multiple animation set(walk, low health walk, run, idle, rare idle) that we can choose from comes to mind, so that we all move and look unique. But, animation are hard to do...
Bunch of landing effects/animation if the landing goes perfect.
Different skill effects depending on our frame/cosmetic.
Also, color palletes like Pandagnome said instead of onesies.
Customizable vehicles that our frame rides so that I can get a nice screenshot for my wallpaper.
Although, this is not actually hope for fashionframe, but a mode for screenshotting.
Pretty sure if there's something like Steam Workshop or Warframe TennoGen for Ember, people would just invent some amazing cosmetics that is more awesome than i can imagine.