There must be at least a few people who have migrated from one nation to another, and among those, there must be some benders. Given this, you'd expect a minority in each nation to be benders of a foreign element, migrants and their bender descendants, but we never see these in ATLA. We have Jeong Jeong (The Deserter), but even he was in his own camp near a Fire Nation colony, and not in any Earth Kingdom settlement. And we have Hama, who isn't as much a migrant as she is an escaped prisoner of war.

I don't have any good headcanon to explain why there are no foreign benders, so my headcanon is that they're out there, and that it's a damn shame we never got to see one. It would have been cool to meet a random fire/water bender in an earth village, or even an air bender but without any of the culture for that foreign bending, as they have assimilated into their new Earth home (being 2nd+ generation to live there). Even cooler if everyone in the village knows about it and doesn't treat them any different.

That the exotic bending we see if due to mixing races.

Lava bending is earth bending and fire bending. That's why Bolin can lava bend, but his earth bending isn't pure enough for him to metal bend.

Explosion bending is air and fire. Sand bending is earth and air.

Master Jeong shows a fire bending power that to my knowledge we never see another fire bender utilize. He extinguishes fire created by another fire bender. I think that his practice and control allow him to not only generate the molecular energy that ignites fire, but also siphon away molecular energy which is an entirely different skill.

That book two of Korra doesn’t exist

My pointless head cannon: Aang the Bloodbender

Aang was good at picking up waterbending from just watching, even if not enough to be a master easily he could still do it. Aang totally picked up bloodbending, or at least thought he did. The idea is not hard to get to once you've seen the premise, and Aang both saw and felt the premise in the fight with the puppet master (blanking on her name).

So why doesn't Aang seem to use it or even know it? Katara. Aang knows Katara wants the practice to end with her so he never tries it, never practices it, never even acknowledges it when he can avoid it.

But I would be surprised if there wasn't at least one moment, one fraction of a second, where Aang didn't look at the full moon and think, "I could. I won't, but I totally could."

Let me be my own critic here and point out the obvious: Bloodbending is a seriously advanced technique that takes a good amount of prior knowledge even for a master waterbender like Katara. Aang doesn't have this prior knowledge. Also there are some techniques Aang just doesn't bother with, like the swampbender's plant bending, so he could have just ignored bloodbending.

But come on! The Avatar saw and understood arguably one of the strongest bending powers and CHOOSE to not use it out of respect for those around him? That sounds so perfect!

That the entire Korra series was written by horny corporate virtue signalers with no actual moral values rather than the same team that made ATLA.

The entire world is just yucky and forcibly westernized for absolutely no good goddamn reason. The characters we get introduced to are shallow, narcissistic, and basic beyond belief. Instead of wanting to learn more about the world like I did with ATLA, I'd be right in the fuckin swamp next to Toph.