I've seen novels that have grown out of one story in a collection. But it hasn't occurred to me to take any of those stories and build on them. They seem very finished for me, so I don't feel like going back and dredging them up.
“You lie awake at 3 in the morning thinking of story ideas. You're online at 8 a.m. on a Sunday or midnight on a Wednesday. It's a job that you never push aside.”
“But in the meantime I became accustomed to the writing life and it would be hard to change now - partly because of the salary cut if I went to my other love, teaching; and partly because I still have stories to tell, even though it isn't all that fun doing the work anymore.”