I like how the show ended, with the world finally at peace, I would like to see if Aang found a way restart the Air Nomads, that would be interesting.

maybe a spin-off might work instead of a 4th season?

personally i think a show in the same universe but without the main characters in the spotlight would be optimal, their stories have already been told and i feel anything further than that would feel forced and it runs the risk of losing the originality that made the original series so great. i think the continuation of they're stories should be left to fanfiction. i think they should tell the stories of maybe the next avatar, i wouldnt really want a prequel because i am more interested in what comes next instead of what came before. one storyline i would be interested in is maybe an adult aang trying to repopulate the air nomads, or maybe the next avatar trying to find a small group of airbenders in hiding so he can convince them to come out of hiding or something, Avatar: The Lost Airbenders?