I would love to see more long hair, especially long curly hair, after all ''most'' Star Trek women have had long hair.
More non vulcan looking hairdos with bangs (assymetric bangs ftw)
More hairdos with decorative elements (especially for Aliens and Orion Slave Girls)
STO already has a nice selection of cute short hairdos, I'm sure my hairdresser will laugh when I bring a picture next time heh
More shades of makeup that don't just tint the red-ish colour. (green should be green - orions and andorians look a bit silly with most shades of pink lipstick tbh)
More black and red haircolours
In addition to this I would like to suggest certain acolades unlock ''special'' costumes, like those catsuits all the ''special'' women of Star Trek wear (like T'pol, Seven, Troi...) and others (''uniform dresses'' á Kess, Kirk's famous tie-around shirt not to mention his even more famous torn chest shirt....ooookay maybe not)
Edit to say i agree that a casual wear would be cool. It would be awesome if we had a ''causal wear'' tab for costume, unlocking maybe at an early rank, which would automatically load if we visited a more causal place (not stations perhaps, but certain planets for sure!) Since this is a war, it does (sadly) make sense that even Orion women have to wear a uniform...but sometimes you just want to see the costumes you relate to the race y'know..)