Having more than one story arc adds depth, complexity, and interest to your story. Story arcs can be subplots or simple little details that you tie together. Be sure to manage them properly, however, and close them, when required, so they don’t leave your reader hanging. That’s a faux pas that shows you’re not paying attention to your own story.
A well-woven story will have several things going on, albeit in the background. Tying characters together is one way to do this. Rather than having random characters come on stage, then leave permanently, tie them into the plot in some way. It’s a small world and coincidences like that are credible. I believe that mystery writers tend to do this intuitively, but it works in other genres as well.
If you’re writing a serial, you can have a lot of fun with this concept. Anyone who has followed your series will appreciate a tidbit now and then that refers back to a previous episode. If it influences the current story in some way, all the better.