I know some variation of this has been posted before but:

I wish we can combine any combination of costumes, and normal gear, each part acting as a slot. I also wish that polymorphs would give costume pieces and a cosmetic race. Each xivkyn morph has different features, so no overlaps there. The only small issue is maelstrom baron and dredguard share armour. (They could be considered different armour so differently dyable. Same model costumes exist already.) Also the skeletons are the same unless you are a beast race (Could be fixed by changing one skellys colour.)