I think that he may have eventually pulled off season 3 if he stuck around and worked on it more, but it's clear that the show was mostly filled with ideas and sketches that he had been thinking about for some time and the first two seasons truly do feel like "Greatest Hits" because of that. Just look at some of the references. He had that dude from like Real World 98 on there....
Also the Wu-Tang skit...other things as well. I just get the sense that Dave had been contemplating on a lot of these things for quite some time and after he used up all of the "ammo", he didn't really no where to go from there. Of course, he also seemed to dislike the ultra fame that he got in such a short period of time.