It's not that complicated. Think of all the metahumans with poison powers or bug based powers. Perhaps Naruto style ninja clans with Kekkai Genkai are descended from those who were 'bug benders' or 'Darkness benders' or the like. Maybe the Spider who bit Peter Parker was a bug type spirit animal trying to share it's power but died from exposure to radiation?
What I meant by complicated is that ATLA put a ton of complexity into FOUR bending styles. Firebending extends into heat manipulation, combustion, lightning redirection/bending, jet propulsion, and energy reading. Earthbending extends into magnetization, lavabending, metalbending, mudbending, sandbending, and seismic sense. Waterbending extends into freezing, bloodbending, healing, plantbending, spiritbending, and steam manipulation. Airbending extends into suction, propulsion, asphyxiation, hypersensitivity, flight, and spiritual projection. Beyond that, they have spiritual aspects, cultural aspects, and modernizations in LoK. It's a lot more than just "the power to control [element]."
My proposed solution is one that is already covered in Legend of Korra: instead of a style of bending for each, the other styles are all connected to spirit possession. In Wan's story, a guy gets possessed by a tree spirit, and it leaves him part wooden. Another guy is possessed by a lemur spirit, and it leaves him part lemur. So that's how Pokemon got the way they are. Most of the originals were regular animals, but had interaction of some kind with a spirit that left them and their descendants changed. For those not covered by that, we could go with possessed humans, possessed aliens, or even spiritually manipulated objects. This would work quite nicely with what you've created.
...like a masterball or specialised Ultra Ball made by a legendary crafter such as
Kurt. And you'd still have to get it to hold still long enough to strike it with the ball, or weaken it enough.
Omg. If Kurt could make a PokeBall capable of capturing Majin Buu, that would be godlike. I wouldn't be nearly as impressed if Silph Co., creators of the Master Ball, did it. After all, their creation can capture GODS. I'd venture you could catch most things with it. Probably lacks the ability to capture humans, but maybe because Silph Co. designed it with that drawback. An illegally modified one would be a different story. But one has to ask: if Silph Co. could make a PokeBall that can instantly and automatically contain the likes of Entei, Rayquaza, Darkrai, Giratina, or ARCEUS THE CREATOR OF THE WORLD, what in god's name did they make it out of? What IS the Master Ball?
Grimm are interesting as they are beings made by an evil presence...
I condensed this for the quote, but I approve of everything you wrote. I've isolated one part below because reasons.
Grimms are found in the wild but it's not clear what
dark force or spirit is causing them to be created.
This idea is genius. Replacing Remnant's God of Darkness with Vaatu is so concise and beautiful, I love it. It also means we don't have to deal with him directly, because the next Harmonic Convergence is set for the year 10,1975 (ten thousand years after Korra's time).
I welcome whatever you have to add from Doctor Who, but I'll want whoever joins next to weigh in on it, providing they have a comparable level of Doctor Who knowledge. For my purposes, I'll need brief explanations and healthy hyperlinks I can use to peek at what you're talking about.