<Not a lawyer disclaimer>
To my understanding, most networks and publishers only really care when you are pirating their works. Writing fanfics of it usually doesn't bother them.
"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard Feynman
Avatar: the Last Airbender is the single most popular cartoon to write for on Fanfiction.net. I have a feeling that if they cared, they would have noticed a long time ago. (ff.n does not allow publishing stories based on works by authors/companies who explicitly tell them "no fanfic"). Not only that, but your stories would be a drop in the bucket to them. I wouldn't worry about it.
It's fine so long as you don't write Squidward torture porn...
Leave that to the show's own writers.
Wait. What. I typo'd and accidentally created a second profile? Fix it.
![[up]](https://static.tvtropes.org/pmwiki/pub/smiles/arrow_up.png "[up]"){kind=icon}
(x4)
Oh boy, let me tell you about a little web site called Fanfiction dot net... Avatar fan fiction is stupid popular there.
Akira Toriyama (April 5 1955 - March 1, 2024).
I've read some of the most amazing Avatar fanfics on FF.net, and the Avatar wiki on wikis has great worldbuilding in its fanfic section. The more I think about it, the more I see a lack of C&D as implied consent :)
The only one they might be more protective with would maybe be ninja turtles. Anyone know about that one? Korra/TMNT crossover fic FTW :)
If there was one world I really want to write fanfiction for, it would be the Avatar/Legend of Korra series, but I don't wanna have Nickelodeon sue/c and d my butt. I assume because they're part of viacom they're ok with fanfic like they are with Star Trek, but can anybody give me more conclusive evidence. Searching on the internet has turned up very little besides "don't ask don't tell".