Mutants & Masterminds? It has a power for controlling substances(Just apply the Limited: x element Flaw), lets you create elemental damaging powers that factor in your strength, lets you create custom sense powers in case you want to make Toph, lets you create damaging powers and afflictions with AoE templates of your choice, lets you create movement powers, lets you improvise new applications of your powers with Hero Points, etc.
I can't think of a thing in Avatar that couldn't be modeled in that system.