More hairstyles. As was mentioned, there is a particular lack of hairstyles that do not cover the forehead, which limits a lot of different rubber forehead aliens including but not limited to Cardassians.
Actual uniform selection for Klingon characters!
Assuming that Wrath of Khan Admiral's Uniform is only the Admiral's uniform and no other options, I'd like to see all of the variations from that era: normal officer's uniform (with skirt option), medical staff uniform, trainee/enlisted uniform, engineering pressure suit-looking outfit, and so on. Also I'd like to see appropriate rank badging for that era uniforms.
Mainly I would like to see more hairstyles for women that are long hair. I mean odd for them to overlook this with so many popular female characters from the series, and none of their hairstyles are in the game. Some times I wonder if cryptic watched the series at all, lol.
In terms of skirts... thats a preorder bonus for those who got the dde... if you didnt preorder that I'm thinking you will be out of luck for quite a while.
Also in terms of mirror universe a dev did comment somewhere before that mirror unifroms are in at launch, although he never said in what form. Might be something you find in game, might be part of the TOS preorder, or might have to be bought in store as that is considered a cosmetic piece.