People have wanted to add trampolines into Minecraft (Not pistons, like, LEGIT trampolines) for a long time. Slime balls have been lying around in inventories for ages. So, I think Notch should put those together and get a slime BLOCK, which will act like a trampoline, bouncing any mobs (Or the player) Into the air 5-7 Meters.
They could be crafted like:
  
If equals a slime ball. And blank is, of course, nothing.
These slime trampolines could be implemented in mob traps, mines, transportation systems (Would they work with minecarts?), or just plain fun. Would this be a good idea?