My go to reference for this is Search Party. The first season is the most engaging TV I've ever seen, the second season is still pretty damn good, but then the show got picked up by HBO and became total shit for 2 seasons. Like god I fucking hated season 4 of that show.
As far as movies go, I gotta go with Shrek the Third. 1,2 & 4 are all great, but what the fuck was 3?
Fringe, and similarly basically any high concept drama without a defined endpoint. Weeds is another one of these. Shows like this can go approximately 3 seasons before self-destructing when they either run out of coherent ideas, exhaust their ability to believably raise the stakes, or a writer shakeup detonates the narrative thread.
Lost is a show that I watched that kind of fits this description.