What specifically did you have in mind for this idea? Cannonballs that are flaming when fired? Cannonballs that catch on fire when they land onstage?
The former is a) the standard option and b) the easiest to implement. A cannon that catches fire when landing onstage sounds much more like a Molotov or a non explosive grenade to me. Though, it could work as a "trap" type of cannonball(like the Rocket Raccoon thing some pages ago), doing double duty as defense and possibly a small combo starter.
And while we're on the subject of cannonballs, here's a few ideas at the top of my head:
-Spiked cannonball: Slower but more damage.
-Cloaked cannonball: More startup but difficult to see.
-Glove/Hammer cannonball: Slower projectile that stops in front of an enemy and whacks them with a boxing glove or a hammer. Trigger range would be maybe a character's length. Would fit the playing dirty aspect.
-Ice cannonball: Less damage but 2 second freeze. Would be a fairly situational cannonball type.
-Explosive cannonball: A timed explosive whose timer increases depending on time charged(going off the possibility of cannonballs being charged attacks). A tap makes it clock in at 2 seconds. Half charge would be 4. Full charge would extend it to 6.
-Scatterball: At the cost of damage, splits up into 3 smaller balls after a small distance and covers more. If you wanted even more range, make the initial split into 6 smaller balls and reduce damage farther.
-Restraining cannonball: With increased startup time, fire a cannonball that traps the enemy in a net or a tied rope. Great for team play.
These are all imperfect ideas, so do alter them if you so wish.