This reminds me of the book Flatland which is about what a 2 dimensional universe would be like and the experience of a 3 dimensional entity visiting it. Great read! https://en.wikipedia.org/wiki/Flatland
Say, a powerup temporarily turns the screen to 3D, with the ghosts only perceiving the 2D slice. Pac-Man might be a cone that can 'shrink' to squeeze around ghosts, can jump through 3rd dimension to 'teleport', can move back and forth between different 'levels' floors in a building, etc.