The thing is, as we cannot stress enough, Avatar was designed right from the start to be a trilogy. If another season were to be produced, then it would have lost something. It might be small, barely noticeable, but it would have lost something. Although there are and always will be a few kill-joys, the general consensus is that the finale and the way Avatar ended was perfect.
To fill in the time, a lot of us have turned to writing fanfiction. With all this passion, my personal belief is that Avatar would actually do very well having a series of EU books written. I am not talking comic adaptions - I am talking full length novels. I'd definitely read them