“It’s just a matter of when,” MacFarlane is quoted as saying by Entertainment Weekly. “It’s hard to do that while you have the series going on at the same time; I think that’s why it took The Simpsons 20 seasons to figure out how to do it.”
The Simpsons Movie was apparently quite a touchstone for MacFarlane while considering how to do the Family Guy movie. MacFarlane praised the film but expressed a desire to correct some of their mistakes with his own outing.
“We do know what the Family Guy movie will be. The Simpsons movie, I thought, was hilarious, but the one criticism I would have is that it’s a story they probably could’ve done on TV. There could’ve been an episode that had that plotline,” he said. “That’s the challenge with animation. You pretty much can do any story you want, so what is the reason for the movie? We finally hit on the answer to that question, and it will be something that would be impossible to do on TV.”
He added that he's already got a plot in mind, although he can't tell the students what it is. It's odd, of course, to think of something that would drastically increase the series's scope, since MacFarlane has explored the Multiverse before--seemingly about as big as you get.