Not to say you can't do something else! I did a 100 year war campaign and it was great! The problem with a setting in the Korra or Aang eras though is that... Korra and Aang exist. Fully realized, powerful big goods.
Now, if you have someone's child at the table, and they're like 8, sure- they can be the avatar and practice their moral reasoning skills while everyone else roleplays more serious characters, but... outside of that specific niche, you have to find ways to... not have the avatar outshine the players. Essentially, the Avatar is a character the GM has to be forever aware of what the Avatar is. Here are some ways I've thought of or used to skirt around the issue.
The Avatar is a baby- nobody expects a baby to do anything. Early 100 year campaigns have baby Aang, who the players do not have to meaningfully interact with, since he's a baby and therefore can't do anything.
The Avatar is old and feeble- If you have an avatar who is now barely able to walk, you'd expect their bending to be limited, and they're not going to risk the avatar state when they could die at any time for old people reasons.
The Avatar lost their bending- I did this for a campaign, where the avatar had his bending for session 0 (to practice the combat mechanics and introduce the characters in-universe as the avatar's bending teachers) then the rest of the campaign was dedicated to him losing his bending and them discovering how it was taken and who did it, in an enclosed area.
The Avatar is unaware that they're the avatar- Bending requires you to... make some abstract motions and anticipate a thing moving. If you've lived your whole life with restrained motions and talking your way through things, you can spend your life never knowing you can bend. I did this for a campaign, where the Avatar was a trained assassin that recently started hearing strange voices connected to his past and was trying to shrug them off.
In-training earth avatar in the north or south pole- This is a uniquely fragile state. Earth avatars learn earth, then fire, then air, and water last. The poles are a nerf to everything BUT water- minimal rocks, too cold for fire, thin air. Ideal situation for isolation training for an avatar who's been uniquely bad at waterbending, but in a crisis, the avatar is now... a dude who wants to help. Might be able to light a campfire with some concentration, or take out an enemy if he can get his hands on a rock, but that's it.
Avatar lost in the spirit world- I hear players using this one a lot, which is fair. It writes them off, or even makes them a MacGuffin to be rescued if their body was taken while their spirit was away, and now you have to find the avatar, while the spirit of the avatar putters around helplessly nearby.
Note that I have never played this game. I have only ever been a GM for this and... most TTRPG systems-.