I have never seen much American Dad, but I always thought that Seth McFarlane was the only guy really driving the ideas for Family Guy. I guess I never realized that multiple people were working on the writing staff. Still, I think that movie idea is going to be a tough one to pull off without just being crude for the sake of being crude.
That happens with everything that someone takes credit for. There are multiple people working on a show or idea, and it takes the whole crew to give us a finished product. Look at the Muppets, for example. The performers take the bare essence of an idea for a character, but come up with their personalities themselves. look at Oscar. That character went through a LOT of evolution over the years that made the character more complex than the basic idea of some nasty guy who pops up and tells people to go away.
Unless the creators regularly write for the show, they basically guide the writers one way or another if anything. Spielberg didn't create the characters of Animaniacs or Tiny Toons, but rather guided the production, picking out characters and segments he liked.
I wonder how much of FG Seth actually does. I know he does like 90% of the voice cast. I'm sure he does guide things in FG, but he has a whole staff of writers pitching their ideas too. He's pretty disconnected from The Cleveland Show to the point where they had to recast the 2 characters on the show he actually does the voice of when he was working on Ted. American Dad he only really does voices for. Actually, I heard he wanted to end FG after the 10th season, and he kinda had a fight with Fox about the Return of the Jedi episode. Fox wanted him to do it, he really didn't want to, so he just dumped it on the writers.
Still, I have to admit, I liked his stuff a wee bit better when he was doing cartoons for a younger audience, like Johnny Bravo.