Some women's fiction authors seem to do a version of series by means of a subtitle like "A Lowcountry Novel." Some have series names and some use the same term on their titles.
I just listened to a Self-Publishing Show episode with Alessandra Torre and she said she was kicking herself for not tying some of her books together, even in a small way. She talked about having four stand-alone books, all set in Los Angeles, and she didn't try to tie them together at all. If she had been paying attention to Stephen King, she would have seen how a master can work that kind of thing into a story.
I'm doing it myself, with my
Cold Shivers Nightmare novels. They are all going to be stand-alone novels, but they will have small connections throughout the series. In the last chapter of
Shattered Prisons I have my main character from
Beware The Boogerman show up on my MC's doorstep and introduce herself. This kind of thing will happen through the entire series of 40-60 books and I'm sure there will even be opportunities to bring characters together for entire stories and have them work together.
I kind of look at this series like Lee Child looks at Jack Reacher. All books related, but every one a stand-alone novel.