With a theatrical budget they can do things with the animation that they couldn't do on the show. Considering when the show cuts loose with action scenes/full animation it's some of the best stuff with the series, I can see how a theatrical feature might look really, really cool. Like how the Simpsons movie had a bigger "look" to it, and had fuller animation.
A Family Guy movie done like an action/adventure film could be really cool. I just don't think Seth is going to take the TV show, stretch it to 90 minutes and slap it on the big screen. I can't imagine him not wanting to do something cool with a theatrical version of the show. And you know this thing is going to have some major musical numbers.
Maybe he'll do something along the lines of the endings of "Da Boom" or "Not All Dogs Go to Heaven".