What the fuck I was thinking of making the opposite of this topic
The Red Star. It got so, so underutilized and then they just forgot about it. I want to use that (preferably with improved controls) for like, some kind of actual mission.
My favourite one-off concept has to be everything that defines Cappy, gameplay-wise. It may seem like a trite choice, but there's no denying the cap-throwing mechanic was executed really well, both in how smoothly it is carried out and how creatively it is used to manipulate enemies and objects. In addition, I commend it for giving way to the game's overarching hat theme. In turn, this influenced the concept of hat-collecting, which served as a fitting side objective to run concurrently with the game's multicultural and globetrotting nature.
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.