Event/Holiday items
Back items can be transmuted and become a higher level item using Transmutation Stones / Transmutation Crystals, a high level back item, and the event item (obviously picking the appearence of the event item but the stats of the higher level one). In that way they can be used by higher level characters without penalty. I think those are fine as-is.
Smaller bags (like the Trick or Treat bag or Rox’s treat bag) are stuck and probably useless for higher level characters, and thats a bit of a bummer, but it doesn’t seem like a huge problem.