*SEASON 3 SPOILERS AHEAD*
With the intermingling of the different bending nations in Korra, we are introduced to couples from different nations who start families together. My headcanon proposes the question: “Could benders from two different nations create a hybrid bender?” For example, an earth and waterbender give birth to a mudbender. Or an air and earthbender give birth to a dustbender. This was just a theory at first, but then I noticed something. There are only TWO lavabenders. Ghazan, and Bolin. I thought that Ghazan bent earth on on a molecular level, changing the state of the earth, in a permutation of earthbending, not unlike metalbending. But Bolin threw a wrench into the theory. Look at Mako. A firebender. So one of Mako and Bolin’s parents was a firebender, and one an earthbender. We know nothing of Ghazan’s parentage, and it is implied (when Bolin plays the game where he makes up personal details about Ghazan) that he is an orphan. So it’s completely possible one of his parents were also a firebender. I know this is all farfetched but it makes sense. I think they were born with the “lava” gene, passed down through a combined bending energy.
Whew that was long winded.