In a recent interview, Simpsons executive producer Al Jean estimates there's a "50-50" chance we'll get a sequel to 2007's The Simpson Movie. He also revealed that January 4th's alien abduction episode – 'The Man Who Came to Be Dinner" – almost provided the plot line for said sequel.
EW got the details from Jean:
James L. Brooks opted to hold it back as a possible plot for a sequel to 2007's The Simpsons Movie. "Two of the allures were exploring the rules of the new world and the cinematic nature of doing something in space," says Jean. "But then we were worried that people might think it's an idea that's not canonical—it doesn't really happen, unlike all of other 560 episodes that really 'happened'—so the ultimate decision was to air it as an episode," Jean explains. "It just got to the point where if we were unsure about it as a movie, then it would be good to air the episode. And then if we do a movie, we'd just think of something else…. So if you want to know what was thought of a possible Simpsons Movie 2, we just aired it—for free. You can see it for free!"
Oh, canon. Always getting in the way of things.
On the 50-50 odds of us getting a sequel to The Simpsons Movie, Jean offered the following:
Our feeling is that the first movie was pretty successful and we don't want the second movie to be any less successful. And I'm not talking about financially only—I'm also talking about no one wants to do a movie where people think, 'Why did they do that? It wasn't necessary.' We actually think of ideas for shorts more easily than for features. To be honest, there's nothing that I'd say, 'This is what we were thinking we would do if we did a feature.'