Plotting a series of novels that will keep readers entertained and coming back for more is a large task for any writer to take on. It requires planning—even if you usually don’t subscribe to things like outlines and the like. You have to at least know what the world you’re creating is made of, so you know if A) you have enough material to get a series out of it and B) if you have a world that both you and your readers can be interested in long enough to read a series about.
A Whole New World
What makes your world interesting? What’s different about it? If you want to write about vampire pirates, you need to throw them into a world where they can move the way they want to. Sexy elves? Probably not going to take place on current-day Earth (unless it does and we just don’t run across them for some reason).
You need to pick a place where you want your characters to live. If it’s normal, modern Earth, that’s fine. Your work is done and you need to pick where your characters will live. If it’s anything other than that—you have some work to do.
The World is a Stage
Every stage needs actors. Who will play on yours? Think about the beings you want to write about in your series. You might write every book in your series about one character, like the Harry Potter series. You could also create several characters and have them come in and out of your books, like Anne Rice did with her Vampire Chronicles.
This is something you should decide before embarking on your series. Then again, you could write one book about one character and see if that character has enough fuel left in them to make it through another book.
A Reason for Everything
Why do these characters matter in your world? What brings them together, or drives your main character toward their goal?
Your series needs a common enemy, or goal, or reason. A vampire pirate is great, aye, but without a purpose, Captain Blackfang is just floating aimlessly on the dark seas. Give him a destination for which he can set sail, or an enemy he can destroy.
Some series deal with one problem or goal per book and then the intrepid crew members are onto a different adventure for the next. Some series have a problem the characters must face in every book, but these are simply stepping stones they take to reach some ultimate goal. Again, Harry Potter is an example of this, as well as the Chronicles of Narnia.
Write What You Like
At the end of the day, write about whatever characters and whatever world you’re most excited about. Because, if you’re not excited and interested in what you’re writing, your readers will smell you out and be doubly bored with they pick your book up. Be true to yourself and what you’re passionate about and your readers will love the places you take them.