A Wall-E sequel would be rad as fuck. And it would sell. But I actually really like it when a movie is great, and it stands alone. No sequels, no loose ends when it's over, just a singular great experience.
It's also great when it's a nice, solid trilogy of movies that wraps itself nicely on the third movie, and never pushes its luck and goes further. You know, like the Toy Story trilogy and how they just made three and ended it there.