First, a disclaimer: I really don't know what I’m doing and you'd be better off taking advice from someone who does. Because honestly, the more I write, the more I am convinced I really, seriously don't have a clue what I'm doing. But Carrie Clevenger asked me on Twitter about my thoughts on this whole sequel business a while back, so here we go.
In your first book, you establish a world and even more importantly, you establish your main characters. Who are they, what are they about, what do they want, what are they willing to do to get it? Not only did you ask those questions but in the course of your plot you answered them. But that first book didn't tell you their whole story, otherwise you wouldn't be writing that sequel, now would you?
One of the things we do to explore what our characters are made of is something I call escalation. This is basically when you throw your character into the frying pan and have them struggle their way out. And then once they're out of the frying pan, you throw them into the fire. If you're writing a short story you do this once. If you're writing a full length novel, you and your characters might as well get used to being crispy. I think escalation works whether you're writing a stand-alone, the first in a series, or a sequel. What might change from book to book is the nature of the fire.
In Mojo Queen the nature of the fire Roxie was fighting was external. She was contracted to help in a case of demon possession. It wasn't a personal issue. It got personal as she and Blake developed an attraction for each other, but even with that budding relationship it was still just a case for Roxie.
In Red House, the second Mojo book, things are different. Roxie is still hired to work a case, a haunted house this time, but her perspective is different. Roxie has lost her home in a catastrophic flood and her entire life feels adrift and unmoored. Hurt badly by her inability to save her own home, she is determined to save her clients' home, even if she has to go deeper into hoodoo and magic than she ever has before. Because, ahem, this time it's personal.
This time it's more than just plot escalation that's necessary, it's character escalation. By changing the nature of the fire - from an outer conflict to a more inner conflict - the stakes have been raised considerably. Roxie's heart is on the line as she struggles to figure out where she and Blake stand with each other, her sense of self-worth is on the line as she struggles with her abilities with magic, her sense of stability is on the line as she faces having lost her home. With Maple Hill, the haunted house of the title, acting as a stand-in for her own home and other things, everything has become personal for Roxie.
The most important thing about a sequel is getting deeper into your main character. This is their story, after all, and each book is part of their story arc. Part of their journey, if you will. The farther along they get in their journey, the more difficult things should get. The rewards should also be greater too - don't forget that part.
So -> Escalation. Raising the stakes. Getting deeper into your main character's heart and soul. That's really all I've got as far as sequels/series go. I know it's not much but if this is something you're tackling I hope it'll help at least a little. (Further disclaimer: I really had no idea what I was doing with Red House until about two-thirds of the way through. One day I was just, ah, so that's what this is about. I can't outline to save my life.)