I agree with firedrake and Mishell. Write what you love and damn the trends -- trends change, and can change in extreme ways between the time you start writing your novel and the time you finally sell it to an editor. Then it'll be another year or more before it ends up on the shelves. Stick with what you love.
I can tell you my own experiences with historicals. I love ancient history, particularly in the Middle East and South America. I wrote an Egyptian novel set around 1500 BCE. I did get quite a few rejections during the agent search, and yes, many of them were along the lines of "I am just not personally interested in Egypt enough to be the right agent for this novel" and even "We've had success with several other authors writing Egyptian historicals, but we're not looking to add more to our list right now" (so there you go, right there -- following a trend isn't always a good thing!
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However, I did find an agent who really loved the book and who works well with me. It was just a matter of patience and being choosy about to whom I pitched the book.
Now the novel is on wide submission and we're getting the same kind of rejections -- "I liked the writing, but this historical period and setting may be too hard to market reliably right now." I am not worried. It's true that there are few ancient Egyptian historicals out there -- certainly not as many as, say, Tudor England or Civil War USA. But as with the agent search, eventually we'll find the right editor -- somebody who clicks with the story, the setting, and the writing. It's a waiting game, and it requires a lot of patience. It makes it easier to wait because I know I've written a good book that WILL sell once it falls into the right hands. I wrote a book I can believe in because I ignored what was popular and wrote about a setting for which I felt a lot of passion and excitement, and I put that excitement into the book.
Definitely stick with what you love. If it's a good book, eventually it will find an agent and it will sell. Good books are always in demand, no matter what their setting.
(Edited to fix my date error...3500 years ago, not 3500 BCE. Need more caffeine, clearly.)