Thinking of burning zombies turning into skeletons, I just came to think of tar!
Perhaps hot tar could be an earth-elemental spell: You get to cover opponents in hot tar, which get slowed and also get burn damages from the hot tar.
If you then add fire to that tar, the opponent is in real trouble, because tar only burns when it is in liquid form, and then it bursts into a whopping ball of fire. Ouch!
A more benign alternative is throwing a sack of feathers on top of the tar victim and you have a demeaning classic.
