I'd love a Borg/assimilated costume pack. Something like the Seven of Nine costume, but with some bigger/more major Borg bits. Maybe even being able to replace an entire hand with a prosthetic.
Then again, they might not want to make this if it treads on the feet of the Liberated Borg LTS reward. Still, this would let people Borgify races besides human/Klingon/Romulan, but wouldn't give the Borg stats.
More KDF options would be nice. Specifically, finish up the remaining pieces of the Bortasqu' uniform- there's a skirt, long coat, and shoulder pads which are in the game files (accessible in the Foundry) but unavailable to players who've paid quite a bit for the uniform.
Klingon NPCs also walk around wearing robes and greatcoats that we can't use. I know they have clipping issues with certain body types, but that's no excuse for limiting our options.
Vixis' sleeveless armor outfit would be nice, too.
I've probably mentioned this in a couple of other places, but instead of the plasticky yuckiness that is the current Sierra 1 uniform in-game, I'd really like to see the "original" Sierra uniform from the Alpha release of STO, which was presented as the "official" Starfleet uniform around the time of the Hobus Supernova in IDW's Star Trek: Countdown prequel comic to JJ Trek:
According to fellow forumer stoleviathan99, IDW obtained the designs from Cryptic for use in the comic, and for some reason (legal or personal preference of a Cryptic game artists), the "Countdown Sierras" were removed from the Alpha prior to the official release of STO.