what.....

i havent been around for a while and i dont really understand all this.......

Well, they could make another one. With stuff like:

Zuko's mother and the search for her by Aang and the gaang.

Azula regains most of her sanity now she is in prison and has time to think about what she could do, and while there is no pressure resting on her of being the firelord.

Aang looking for surviving airbenders.

Rebuilding the world.

More about the order of the White Lotus.

More about Iroh's trip to the spirit world.

Beating down rebels who trie to free Ozai (and you could combine that with Azula). Or who are just trying to destroy everything by thereselfs.

But the things they should not do is making up complete new main villains. Like and uber powerfull firebender who was 3 times more powerfull than Ozai and is now terrorising the world. That would just suck. They did stuff like that to some other tv shows, and that's not cool anymore. Than you just get a weaker version of DBZ. But they could make some strong rebels, with remaining Fire Nation soldier, but not stronger than Ozai or Azula.