You reckon the simpsons have improved? compare the 4th or 5th season with the new one. The simpsons have gone down, way down. Greoning should never have given up on futurama.
That said, i have very low expectations of a simpsons movie, because the simpson's characters just doesn't have the capacity for a grand enough theme, because nothing changes, except for the death of a very few minor characters. the Southpark movie did, which was the death of Kenny/rise of satan/redemption of Kenny.
The only way a simpsons movie could be any good would be to either kill a reletively important character (apu, krababble, mr. burns) or have a 'coming of age' scenario for either bart or lisa, which would affect future series. the simpsons is too old and too comercial for such a change. A Simpsons movie is likely to be trivial, therefore boring, cause you can't strech out a 30min episode to to 90-120 mins
H.B.Flaubert out.