I think a movie could have worked. Not a 'Movie' Movie, but a TV/ Straight to DVD movie. The Family guy movie is an example of this: not revolutionary, but enjoyable, and something that fans could enjoy. I don't think you would need to go out of your way to bring non-fans up to speed with the characters, because by that logic you would need to do that at the start of every episode, in case someone who has never seen an episode before watches it.
Plot: A trip away would be the obvious choice, but perhaps a little cleched. Um... I dunno, maybe Francis gets into trouble and the family have to rescue him?