firearms are already in the game; however, I thought you didnt want to empower players too much with firearms
Hmm, a cannon might work though. If it has a cannonball, it will shoot it, and it a cannonball hits something, there is a certain chance that nodes around it will be removed. e.g. obsidian cant be destroyed easily, but cobble is obliterated, etc.
Without a cannonball, It can just make a lot of noise. Also, it should be able to shoot a player out of it. I guess, if you rightclick it then you can get "inside" it, and then if it fires, you will get thrown very far. This could be done by creating an object that get shot out of the cannon, and attaching the player to the object. Anyhow, that might be kinda fun.