Modern Myths were fun, quick, stories to write. There are no more at present, but there may be more in the future.
When people think of a series, longer stories often come to mind: stories like Harry Potter, Narnia, or Percy Jackson. But who says short stories can’t be in a series? Just look at the Mercy Watson books for one example of a short series.
Take one of the stories you’ve written before. If you haven’t written any, this is a good place to begin. Think of your different characters. Think of the different troubles they could get into and out of together. Mix and match, if you like; take some characters from one story and some from another. Now, write a set of stories with those characters. The stories don’t have to be long, but there should be something in common with all of them, even if it’s no more than that they have the same characters. You can use the original story as one, or you can use it as much or as little as you wish.