I would like to suggest an item that block xp gain for the pokemon using it.
It would help in Trainer Tower, keeping some useful pokemons in the same level and usable for training others pokemons later.
Also I think helpful keep some pokemons in a low level. For example, I always use the same hunt team (sync, trace, frisk, false swipe, arena trap and a shiny togepi for lucky :D). The arena trap digglet I like to keep at a low level, so I'm able to hit Abras a couple of times if it's breaking the pokeball. An xp block item would help to keep in a low level when some XP party buff is active (like it's today).