How about a stage building itself?
For example, there are several tools on the bottom, palette on top, and a mouse cursor, which seemingly does nothing but move around the field, moving towards different positions. Sometimes the undo-dog goes out of his box, but he's mainly harmless.
The stage stays empty, until you attack the mouse cursor, after which it will for a certain distance draw a line. If you picked the line tool (by attacking the ground), it will draw a streight line, on which you can walk. Fortunately white counts as just erasing.
The palette also can switch to stamps, in which certain entities can be placed on screen, doing different things. For example, the baby Yoshi, when touched from the front (having the classic Yoshi sound, pitched based on its height), will swallow you, while the Mario head functions as a bumper (with a piano sound). The cat, dog and pig can cause small damages. The starman, when attacked (it falls rather quickly) will cause a short hail of sprites in the background, with some sprites spawning on the forground.
Meh, I'll make an example later. I had some other fun stuff in mind as well.
