I finally got my slime fully working the way I wanted, both on the art side and the code side. It jiggles when hit, stares at enemies in an aggressive way and jumps at them.
Here's a Slime I tricked into doing a circus act (the ball is a decoy prop that triggers it's attack behavior, without which it just sits there):
Here's the newest sprite sheet:
The slime is now published as part of my game engine. If you're interested in that, it's called Big Block Engine. If you want to use this sprite, it's license is CC-BY-SA 3.0. For the attribution part, I'm Richard Lewis and if possible please share a link to my game engine when you credit me.