Not yet a playable game, but it demonstrates some basics for a Mario-style game:
-Jump on enemies to squash them.
-Being hit decrements the number of lives, then makes you invincible for a second or two so you don't lose more lives from one hit.
-Touching an enemy from above kills it; touching one from the side or from below kills you.
-Add more enemies. Just clone the mouse a few times.
-More kinds of enemies. A bat or bee, maybe.
-Add platforms. Just draw them on the stage in that dark gray color the floor is.
-Probably needs some work to bounce off the bottom of platforms or jump through them sanely.
-Start with fewer lives? 9 is traditional for a cat, but seems to make a fairly lame game.