At this point I feel like Cosmetic DLC would sell well and be very appealing to a lot of people. Primal added some very good new outfits, but there is a lot of room for just adding more outfits, particularly to the older cultures. It's literally turning me and several friends off the game that we can't make the sort of characters we personally want to play.
Everything past this point is just ideas for what could be added based on my and my friends tastes. There are certainly far more ideas than this (feel free to comment your own!)
Female Hairstyle: Bushy Pigtails
Female Hairstyle: Long Straight (Hime Cut or similar)
Feudal Outfit: Full Plate
Reaver Outfit: Armored Ballgown
Helmet: Glasses
Helmet: Monocle
Helmet: Dragon-themed Hairclip
Helmet: Headdress
Weapon: Rapier
Weapon: Gladius
Wings: Fey