There doesn't seem to be a megathread for costume part suggestions, so I figured I'd just put all the ones I thought of here, in no particular order. 😄
More casual shoes would be nice, such as sneakers with shorter (or no) socks, shorter combat boots, and sandals.
More types of shorts for male characters, since they only really have boxing shorts. (I know you can just use knee length pants for a similar effect, but it's a bit long for my tastes)
More hairstyles. If it's possible, a good place to start would be porting male hairstyles to female characters and vice versa.
Splitting up some more accessory categories would also be really useful:
Splitting up head accessories into a head section (horns, halos, headbands, antennae), an eye section (glasses, goggles, eyepatches) and an ear section (earmuffs, earpieces).
Splitting up mouth accessories into a jaw section (mandibles, chin guards, beards), and a mouth section (cigars, mustaches).
Splitting up shoulder accessories into a shoulder section (pauldrons) and a neck section (neck chains, ties).
So yeah, that's everything I came up with that seems possible. Let me know what you all think.