I have even written short stories about some of these characters... yes, I'm THAT much of a geek
I've often worked characters that I never got a chance to play or that didn't fit in to the current game in to stories I worked on for other characters. In a few instances, I've even merged characters together, including one such incident where it was almost literal (a clone of my main Star Wars character and three other DNA "donors", all of which were based on characters I never got a chance to play, and the clone had a mixture of some memories and skills of each. Not expertise of any skills, more like a jack of all trades with a little knowledge of several skills).
I've also used a fair few of the characters I only half finished creating as background characters in other stories as everything from a bartender, a wench, the minions of the main baddie and so on. My goddaughter goes in to laughing fits when she notices a character she remembers me working on for one thing ending up in a story about another.