Don't know if this would be useful but shouldn't be too hard to do or cause any performance issues.

Examples: You kill a spider and it drops a noteblock placed on the ground that it was on. You kill a creature and it "freezes" into a stone block. You mine a stone block and 5% of the time it turns in a gravel block.

Dangerous if users set this at 100% stone to stone, you'd never be able to mine it but possibly useful to if tool is set to "AIR" to stop harvesting of stone with your hands.

This is the replacementblock feature?

Yup, pretty much implemented in 1.8. Still not happy with creatures (would be best to drop two vertical blocks for creepers, two horizontal ones for cows, etc) but that's a different issue.