Cannons sound a lot like the snipers. That's not necessarily a bad thing, because it does add a 'dodge this' element to the game, but here's an idea to differentiate the two (unless you already thought of one).

Have the cannonballs adhere to gravity (unlike SMB3, e.g.)... It comes back to the ground, then it start bouncing toward you (if the initial shot didn't get you).

Another idea: these bouncing cannon balls, you can jump on top of them as if they were a platform. Maybe you could jump on one as it left the cannon and take it for a ride over a tall wall. Or, when standing on it, you can pick it up (ala Mario 2 for NES) and use it as a weapon against other enemies (or a boss).

Just some ideas. ;)

Yeah, I might go with one of my origional ideas to have the cannon fire upward, and the shot travel in an arc. Jumping on the bullet would be really cool, but it might not fit well into this level (next one perhaps?)