A relatively renowned wizard once said, “Death is just another path that we all must take."
Your player’s beloved character dies, he scrunches up the sheet after desperately attempting to be resurrected. After the gaming session, go and grab that sheet, un-crinkle it and put it away for safe keeping.
Once you have a handful of dead player characters, wait for a quiet gaming session. Let’s say the current PCs are in the usual tavern drinking mead and regaling the local populace with tales of adventure. In a dream, a voice from the past enters the PC’s heads. Next thing they know, you unfurl their old character sheets, resurrecting previously dead PCs. Or have you?
The dead PCs might be stuck in purgatory or hell! Whatever comes to your devious GM mind. This could be a great one-shot evening; the PCs have to find a way to their specific version of the afterlife. Maybe a demon or evil god trapped their soul when they died. Maybe the dead PCs are on a different plane of existence and are desperately trying to get back to the prime material plane.
Another idea that crossed my mind is bringing back PCs that everyone detested or the guy that was always the comic relief but died horribly in a dragon-taming accident. You could even change your usual gaming system for this, take the old sheet and convert it. Whatever you come up with, this could be a fun way to breed new life into long dead player charcters.
Wish you had one or more of your old character sheets from 20 years ago? Me too. Now's your chance to put them into a collage, frame it, and hang it on the wall of your gaming area 20 years from now.