Glad to hear you're doing well!

I think the choice of story/setting will ultimately have to depend on what you as the GM want to accomplish/see through this RP. These questions aren't really something you have to respond to here, but just to consider: What is it about the Avatar world that's got you hyped to run the RP? Are you looking forward to descriptions of bending battles and that distinct Wuxia flavor? Or was there some aspect of the Avatar universe's storytelling or world that you really want to immerse yourself and the players in? For instance, did you find a lot of meaning in the emphasis the series puts on character development (every member of the main cast except arguably Toph changes significantly either in their own perception, or in how we the viewers perceive them, from who they were at their introduction)? Or did you really love exploring the fantastical landscapes within the world---the Earthbending rollercoasters in Omashu, the Three Walls of Ba Sing Se, the icy fortress of the Northern Water Tribe, etc?

Which of those things you want to put in front of the players, and in which proportions, should determine your bare-bones plot structure. If you want world-building, you'll need to implement a "journey" of some kind so that we travel a lot. If you just want high octane action, you might want to give more thought to the enemies we'll be facing, how they're organized, and how to make the Players want to kick their faces in.

Then, the rest of this is just my own personal opinion---feel free to ignore it if it's not conducive to your own GMing style:
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The meat of the plot itself should be adaptable enough that you can tie it into your character cast and link up their backstories. For instance, if somebody made a character whose parents were killed by Firebenders, then maybe the Firebender who did it needs to be an underling of your Big Bad OR the Players need to HELP a village of innocent, second-generation Fire Nation Colonists against cruel Earthbending Bandits---both scenarios force that Player Character to deal with the trauma of their past in some way. 

Another example: If someone makes a character who's struggling with the morality of using Bending Arts to kill, then maybe you make a recurring antagonist who's a sadistic assassin despite having no Bending Arts of their own, so that when the Players clash with them it can give the struggling character a means to further their development.

Beyond direct character contrasts between Players and NPCs, though, you should also try to build off what Players give you in their Character Backstories. If one Player makes an Air Bending Thief, for instance, and in their backstory they talk about how they got kicked out of their Air Temple and turned to a life of crime, whereas another Player makes a Non-Bending Bounty Hunter who's a Master Swordsman, then maybe you should DM the two Players and see if they'd be willing to establish a link between their characters that can effect the roleplaying and team dynamics---Maybe the Bounty Hunter once caught the Air Thief, but after being turned in to the authorities the Thief had to take some desperate measures to break out, like breaking the one Air Bending tenet of non-lethality they still valued. Now that death still haunts them. Or maybe the Thief stole a family trinket that was very important to the Bounty Hunter, and ended up selling it, so now the Bounty Hunter can't kill them/take them in until they find out where/to whom the heirloom was sold.

Similar strategies to tie your plot and world building elements to your Players not only keeps them invested in your game, but can give you a jumping off point as a DM. It's so much easier to come up with ideas for how to move a story forward if you have elements you can use as a springboard, versus coming up with new stuff out of the blue whole cloth.
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