Normally my side characters take two paths.
Either I write some bonus chapters and write short stories about them, or I'm writing a book series and I try to show the surface of my side characters that will have their own stories in the next books in the series.
For example, I'm working on a novel series for hard copy called "Straight to fairy tales," Which are stories about two men who, one or both of them, think they're straight until they meet each other.
In each book, there are at least two or three side characters that play a small part and they eventually get their own book in the series.
Maybe the first option fits better with what you wrote.
Another thing I like is to play "What If" with my characters. You know, because sometimes you finish a book and after a while you just miss your characters ?
So I write a bonus chapter and put them through a "What If" situation just to bring them back to life once more.