A puzzle game. You have several different characters of different sizes. The puzzle grid itself is subdivided lots of times (4?), creating “half-movements” for certain characters and such, creating openings only some of them fit through.
You basically get a Sokoban/sliding puzzle where pieces are different sizes, with some extra powers/special rules/spice.
@IDEA: Maybe there are multiple pieces of one size; pressing their specific button/moving one will move them all, if possible.
@IDEA: The other more obvious idea is a shrink-ray/growth-ray puzzle, or shink-grow your platformer, but those have been done by a thousand game devs before.