this may sound weird, but Avatar The Last Airbender (Spoilers Ahead) should have got a full season of zuko and suki on team avatar. like they did the best job with the time they had but its even referenced in the show where each character essentially had a life changing fieldtrip with zuko to make him fully part of the team. and then the finale happens.
and suki in all future media was shown as a full fledged member of the team. but she got very little interaction with the team beyond sokka.
I would have another season to fully cement zukos alliance change, give suki far more development with the rest of the team. also give toph some kind of purpose or actual end to her arc. maybe a reconciliation with her family. make the lion turtle/energybending thing less of a last minute Deus Ex Machina. (like i kinda wish the energybending involved the whole team rather than just ang.) and finally, make Azulas descent into madness feel less abrupt. oh also i dont know, actually explore the zukos mom mystery which they dumped onto the comics.
also, 1 season per element. water, earth, fire, air i mean air as the final season is a sorta stretch. but its where ang learns a peaceful way to stop ozai, which is what airbenders are all about, peaceful resolution. and its like the air is getting cleared of the fire nations smoke. idk i just want the books to match the intro.