Dummy objects is a suggestion that has been requested for a while now, but I wanted to expand it a bit more and see how it could be realistically implemented, without having to changing the way custom tilesets work.
So to start, please look at this concept:
All power-ups acts like the Super transformation and are attached to an specific status (starting from status 13 to 20, or maybe another range of numbers). They all have their own graphics. In the power-ups hierarchy, they are above the mushroom (at the level of Fire Flower and all other power-ups). There is a peculiarity of the sprites though, they are calibrated by the mid of the sprite instead of the top-left corner as the other power-ups. This allows flexible sprite customization:
They follow the same logic as a normal helmet/vehicle (like Coin Box but without dropping coins). They have their own state (from char(1).itemslot -21 to -24) and their unique effect when the player get hurt. The reason why they are called "Custom Vehicles" instead "Helmets" is because they can also be used to create custom vehicles, like Shoes or Clown Cars!
By default, when the player pick one of them, they attach to the head of the player. We can customize their behaviour through char(x).itemslot.
Unlike the custom power-ups/vehicles, they are all attached to an unique graphic (dummy-npc.png) and an unique effect (dummy-effect.png). They do nothing by themselves, they act completely like an empty custom NPC. They are grouped in tiles by 4x4 to organize them better. There are 80 slots in total (and if it's decided to implement a third slot, they would be 128). Their IDs are a reserved range (starting from 400-500 would be a good idea).
They do not have a graphics for their own (i.e, there are no graphics for npc-401, 402 and so on) as they are already linked to dummy-npc.png.. This all applies for their death effects.
Without further ado, here's a download link with all the needed resources:
We have always had a way to customize NPCs, but at the cost of always replacing existing ones. As someone who likes creating NPCs, it's always hard to decide which one will go, not even mention the more you create the harder it becomes, as their IDs shouldn't be repeated in order to use them simultaneously. This suggestions aims to solve this problem! As for power-ups, this system would also provide reliable way to get custom power-ups working, as they currently can only be applied in a per-level basis, not in episodes. The idea of custom hats came after seeing the new trailer update for SMM2.