1. I think I could fill this list entirely with "glamour wardrobe". See Rift, WildStar, GW2, and WoW for some good examples.
2. Instanced housing with improved customization. See Rift and WildStar.
3. Non-consumable dye system. See GW2 and WildStar.
4. Multiple dye channels. See Rift, GW2, WildStar, probably others.
Honestly just #1 would guarantee my sub forever.
It's not unprofessional to see what works well in the competition and incorporate it to make your product more appealing. That is literally why WoW became so successful - they take what works (and often improve upon it) and incorporate it into the game.