Isn't that how it seems sometimes? Writers start with a plethora of muses, who whisper little ideas. Then they all merge. We find ourselves writing the same kind of stories over and over. Not that that's bad. Whatever works, right? But writers need to challenge themselves. If you get stuck in a niche, eventually your stories will be predictable. Once your muse turns cannabal, gobbling up all your other muses, you're eventually left with one muse telling you "write this story with these elements". My fantasy muse gobbled up my thriller muse. Now she insists that all I should write are fantasy thrillers.
Well, it's time to seperate them. Muses aren't like humans. Even when they turn cannabal, you can make them cough the poor absorbed muse up (eventually). Doesn't mean they're going to like it, though! Here's the best method.
What do you write? For example, I write (as I said) fantasy thrillers. In order to get Fantasy to cough up poor Thriller, I've decided to take on side projects that involve Fantasy and GLBT muses. Don't worry Thriller, you'll be out of there in no time!
But why seperate them? Well, as I said: Predictability. Sometimes it's good. It gives the reader some assurance that you aren't going to shock them with things they don't want to read. But it also means that you're eventually going to get bored. There's nothing sadder than a writer bored of their own stories! So my advice, if you find yourself writing the same stuff over and over, is to do what you're worst at. Bad at fantasy? Write a fantasy! It doesn't need to get published (unless in the end you decide it's much better than you thought it would be) but it will, in the long-run, give you better scope as a writer. Keep your primary muses, but adopt a few others as well. You never know when certain elements you think you're bad at will come in handy in your story.