Hey, that last one could work as a movie if not a TV series. Chuck Cunningham comes back.

They could reshoot a few scenes from the TV show with lookalike actors, have Chuck play out his final bit, and then a few more scenes from the show that make it clear that no-one remembers him. At all.

Now, think about it: Extra-terrestrial aliens are canon in Happy Days (Mork and Mindy was a spin-off), so it’s possibly some other alien race abducted Chuck and caused everyone to forget he ever existed.

Depending on what time period he comes back to, this could be played for laughs or for existential horror.