I'll try to avoid ranting on this subject, but I don't know how successful I'll be.
I just saw a sequel to a movie. The first movie was good--it had (1) quick, vicious action, (2) an uncertain but likeable demigod hero, (3) an obligatory love interest that managed to avoid most of the 'damsel-in-distress' cliches, and (4) a subtle but inarguably present pantheon of characters who ranged from good but self-centered to innately evil.
The sequel, on the other hand, had (1) quick, vicious action, (2) a demigod hero who hadn't grown as a character (no offense to the actor, as he carried the part strongly, but making someone a parent does not automatically imbue them with character growth), (3) the same love interest as before, but zero tension between them, and (4) a blatant pantheon of characters who were either good or evil...the only question was, which of these characters were redeemable.
My point is this: whether you're making a book, a movie, a graphic novel, or any other medium of story, please make sure that a sequel deserves to be written before you commit to it.
What makes a sequel good? Here are a few elements that matter to me:
A. Character growth: Unless your story works well because a character never changes, you need to have some evidence of a character growing and changing somehow.
In my completed story about the Greek pantheon, the mostly-human heroine changes even within the first novel. She starts out as exasperated but timid. She would chastize a nymph for coming to her apartment uninvited, but never dream of denying any of the gods or goddesses. By the end of the first novel, she is arguing heatedly with the entire pantheon. In the sequel that I'm working on, the heroine is established as a strong character, but now the gods and goddesses have more respect and less patience with her, due to what happened in the climax of the first novel. That impacts their interactions with her, creating a different ambiance for the second novel.
B. New plot: I know that some authors make a very good living at writing essentially the same story in each of their books.
However, even the most formulaic of authors has to change basic things about their plot, or risk boring their audience. For example, my story about the Greek pantheon will be a mystery series. While the premise of each book will remain the same (i.e. the heroine must search for answers on behalf of the pantheon), the goal of each book changes (i.e. the type of problem to be solved by her alters).
C. Comfortable constants: This may sound counterintuitive, given the ranting (sorry, I know I said I'd try not to) that has gone in under points A and B, but each sequel needs constants.
Having constants is important because, while the characters grow and the plots change, you need something for the readers to hold on to as 'normal' in your story. For my Greek pantheon story, those constants are that the heroine always lives in the same, small apartment over a bar, and her job as a teacher. Constants help your reader get their bearings quickly, so that they can proceed to enjoy the rest of the story. If you change absolutely everything at the beginning of your sequel, it will take the reader some time to adjust, and they may not like or enjoy the changes.
A final note: I love reading a good sequel. The previous three points are merely meant to illustrate some of the elements needed in order for your sequels to be worthy of the readers' attentions.