F.L.U.D.D. from Super Mario Sunshine. This simple device adds a lot to Mario's moveset, granting him a ranged attack, extra opportunities to stay in air, and enhances his movement. Some people would lament this device because of what it removed (e.g. long jump), but the game that introduced it made Mario a lot of fun to control.
A future game could polish the level design around the FLUDD and give it some improvements. After all, Luigi gets to use the Poltergust in more than one game, so I think it's fair that Mario should be able to use the FLUDD more than once too.
Thank you for reading.