QUESTION: I have just finished writing the first draft of my novel, now given to beta readers to test it out.
In the meantime, I am starting a new one, but all my inspiration seems similar to my previous work. Perhaps I am too absorbed in that type of story.
The things that are the same are the team composition of the antagonists, though in my new work they have different behavior and abilities. Also, my new work takes place in a similar fantasy world and has a similar magical system.
First, congratulations on finishing your novel. Of the many who start a first work, very few finish it. Well done!
The truth of the sameness will be in the final products so it's hard to say.
Some very successful writers write the same story and characters with variations over and over again, and some readers don't seem to mind it. Others do.
Each character should have a specific role in your story, and he/she should be written to fit that role. If you want to shake things up with the casting of those roles, you could try what Hollywood calls casting against type. For example, make the bad guy's second in command a charming goofball who has a hidden sadistic streak. Or switch genders.
You may want to do a few major changes/surprises to your world and magic system, but a massive overhaul isn't necessary if the world and the magic fit your story. Or you can set your story in the same world during a different time period or a different part of the world and not worry about the sameness.
One thing it's good to remember is that, these days, a reader will find one of a writer's books, and, if he enjoys it, he will buy the next book by the author immediately and read it. So you want to offer both consistency and surprises.
As a career move, writing similar books is a good thing. Many readers are like kids with a bedtime story. They like what they like, and they want the same thing, but different, each time from the writer.
Successful authors who want to write a second series move laterally by writing subgenres that their main readership would enjoy. For example, Jim Butcher’s extremely popular Harry Dresden series is urban fantasy, but he's written a traditional fantasy series which many of the same readers read, and he's just started a steampunk series.
Then there's writers like me who write all kinds of genres from science fiction adventure to paranormal romance. Many of my readers don't follow me so I have to fight for every reader I get when I switch genres. It’s not a good thing career move, but it keeps me amused.