Well… I don’t know what it would be about, but if they did it right…
The entire first half-hour of the movie would have NOTHING to do with the actual eventual Real plot, being just some strange convoluted way to introduce the subject instead.
It would have lots of real celebrity voices.
It would have even more fake celebrity voices.
Some of the fake celebrity voices would be real celebrity voices, or possibly the other way around.
In addition to these celebrity cameos, it would have cartoon character cameos. I’m thinking Fry and Bender here, among others.
It would have an actual, true, real-life honest-to-goodness 1950’s-style Pre-Movie Cartoon, featuring Itchy and Scratchy of course. The pre-movie cartoon would be completely unrelated to either the fake plot or the ‘real’ plot. And the movie poster of this cartoon would appear in the background somewhere, later in the movie.
Homer would get hurt.
Marge would nag.
Lisa would be too smart for her own good.
Maggie would ALMOST talk, but not quite.
Bart… well… he’s Bart.
Disco Stu would get run over by a tank, never to be seen again.
This tank would be driven by Poochie.
There would be a 5-minute scene created for the sole purpose of cramming in an appearance by every single character ever to appear on any episode of the Simpsons ever.
There would be more than one version of the movie made, as in the horrible failure “Clue”. Two reasons for this occur to me at the moment…
One, for the opening credits. The traditional ‘different openings’ has to be worked in somehow, and one way would be to make four-five of them and distribute them randomly. Or something.
Two, the location of springfield could be a major plot point, and finally be revealed. This location would be different in each of the different versions, for the sole purpose of keeping idiots like us arguing about it on the internet for years to come. q;}
Three, the joke “Hi, Opal” would not be used, in any of the versions.
I feel ashamed using it here. 
Probably a good thing I’m done talking.