One of the good ideas that came out of Pokemon Super Mystery Dungeon was the concept of Looplets and Emeras.
Basically, for those not in the know, a Looplet is an item to be carried by a Pokemon which has anywhere from 3 to 6 (or more*) slots. These slots can be filled by placing Emeras in them. Emeras are special jewels found throughout dungeons, which grant various temporary stat boosts or other beneficial effects. They disappear once you leave the dungeon.
However, some Emeras, such as Big Ears or Telepathy, wouldn’t work, as they require the existence of a mini-map (not present in PMU) to show you where all the enemy Pokemon and items are, respectively.
*An item known as the Emera-up Orb can add slots to a Looplet (up to a maximum of eight slots per Looplet).
I’d like to see these implemented in PMU sometime in the near future.
Soon™, perhaps? XD