As one may know that there are various rings you can equip to change your skin to that of a Monster/NPC. (The CashShop Panda Rings, etc)
How about customized items that allows the same function, but can be configured to be any given monster/applicable NPC instead, any added stats/options are also optional.
Sure, item model and entries wise it can be a bunch of reskins of the existing rings, but one can always change their model and description into something else since rings don't show outside of inv screen anyways.
in S17, the high-end Sets kinda made Custom Sets obsolete, so I'm thinking if a WoW styled Transmogrification system would be better fared. Combined with a custom monsters/NPCs system (which I heard is already a WIP?) I think this can be emulated well.