I agree that there should not be a season four telling the story of further adventures of the gaang. But I am all for a new series with new characters that explores parts of the Avatar world that we have not yet visited. The Lord of the Rings and Star Wars (the novels, not the new movies) stand out as shining examples of how franchises can continue thrive, as long as they stick to the same world that the fans love, but give it a new story. I posted about this on Datruth's thread, so you can read about that there.
I think we should scrap this petition and come up with a new one for a different series set in the Avatar world. Either way though, we'll have to wait for a while because Bryke have confirmed that they won't be doing anything Avatar related until after the movies.
And speaking of the movies, could everyone please stop complaining about how bad they're going to be?!!! Granted, it's going to be tricky to pull it off, but Bryke are going to be working on the movies, and I have faith in the fact that if it is possible for the movies to work, they will find a way. Far from being the "final nail in the coffin", these movies could well be what saves Avatar from the grave. If they do well financially, I find it almost impossible to believe that the original series won't be brought back in some way, either with re-airings of the original episodes, or possibly the creation of a new series, or mabye even a series of novelizations. Who knows? Certainly not you or I. So lets just cross our fingers and hope for the best.