Paper Mario
The addition of standalone styles in Super Mario Maker 2 has opened the floodgates of speculation. Paper Mario could work as one of the standard styles, but it would be best to implement it as an extra style.
There are far too many characters to just let it blend in with the rest of the Super Mario games. The partner characters could act as power-ups throughout the levels.
Giving a player the option to create obstacles for specific partner combinations would be so smart. The extremely diverse attacks of the bosses from Paper Mario are sure to add to the platforming fun as well.
It would also be interesting to see how they could implement Mario’s hammer and other attack or healing items. A combination of platforming and RPG mechanics would be an interesting way to implement the style.
Turn-based battles could be initiated by jumping on an enemy. You could collect items through the level to be used in battles. Then you could finish off the level with a huge boss battle.
There’s an inherent difficulty in trying to translate an RPG to a 2D platformer, but it’s not impossible. If they could pull off the Paper Mario style while remaining faithful to the source material, it would be incredibly impressive.