What if it lets you transform your permanents to ash which is then delayed for 2 or 3 rounds... That gives the opponent ample chance to use CC.
Another would be to put the ash from the destroyed creature / permanent into your deck instead of directly into play... that would give fire a means of prolonging deck-out as well. It should probably not be allowed to target pheonixes or ash though.
A final odd idea would be to have combustion burn up your own creatures and permanents to increase its damage dealt (maybe based on the cost of the card sacrificed)... that would also fit the combustion theme... though this is starting to deviate a good deal from your original idea.