There's a type of series that are called Parallel Novels. Where the main character isn't consistent in all of the books. Where the setting and world details are the same, or the the worlds are all in the same galaxy. And the MC of one book is a secondary character in the other.
I am totally in love with the idea of these novels. I love series, and getting to know the world and characters better. I love parallel novels even more because I get to see more of the world and meet more of the characters. Not just the world according to one MC.
Sherrilyn Kenyon does parallel novels well. Her League and Dark Hunter books are excellent examples of novels with the same characters and world but with different people as Main Characters. Melissa Marr is also really good at parallel novels in her Wicked Lovely series.
In writing my book, I have developed an amazing world. At least I think so. I had two series planned one is four books with the MC of the first book and a second three books with the first MC's son. About a year ago I had an awesome idea for a scifi/steampunk YA adventure. I began to work on it in my head and jot down notes. Then I realized this was an extreme prequel to my series and was on the same world.
I refused to accept it for a long while. I almost felt it was cheating. But after a few months I realized that indeed it was the same world just hundreds of years in the past. Then I also realized that with this book I had an opportunity to create an amazing world and have a lot of fun with it.
Since then I have added another trilogy to this same world, and two stand alones. (at least in ideas and plotting out books) As the idea of the story became parallel instead of series, I became more excited. I also have a few novel ideas in an Urban Fantasy world. I'm excited to get working on them too.
So what makes it a parallel novel(s)?
Anchor characters and anchor worlds.
Anchor characters are a character that is seen in all of the novels. Or you might see a handful of characters throughout the books but there is always at least one.
In my Emergence world there are three main characters, one in each series/trilogy. But my anchor character(s) isn't one of them. He is a secondary character in all of the books though. He's a genius inventor and entrepreneur from Australia. He is the character that links all of the books together. I also created a character that although he is almost non existent in four of the books, (but is mentioned in a round about way) is a larger character in the others. He IS is the fix to a universal plot hole in all of the books.
Another type of parallel anchor is world or setting. If you have a world that is unique and interesting you might want to revisit it again and again with different characters.
While two of my series books and one stand alone in the "parallel universe" are based on Earth. The other trilogy/series, and the one extreme prequel is on another world.
In this way, everything is connected by either world or character and you get to enjoy many different places and characters as well as keep up on some old favorites.
Give them a try, I hope you decide you like them.