For me Once Upon a Time had the same trajectory as True Blood. Both shows created a fictional town with real people, adding a fantasy twist: the characters of classic fairytales in one case, vampires in the other.
In both cases, the pairing was enough to sustain a lot of seasons but no, they had to cram it full of everything (Other Disney movies and different sources in one case, all kind of mythological creatures in the other) until the whole thing collapsed and by the end was watchable only out of affection and habit.