Syvwulf
Official Terrarian
Various different armor sets have visual effects, such as giving the player an afterimage when moving (Shadow Armor) or giving them a pulsating aura (Hallowed Armor), wouldn't it be cool to have an NPC sell these effects as you collect armor sets that have them, and use whichever one you want with whichever armor set? They could maybe even be dyed to add even more customization. I'm not sure if this is the right place to post this, but I couldn't find a Vanity section of suggestions so I assumed it would fit best here.