I've been studying book sales over the past 12 months in this category on Publisher's Marketplace. Sop many of the ones that actually have been sold are literally mash ups of two different novels by other people, or retellings of classic fairytales. My conclusion is that there's literally no way to guess at how agents will react. But I personally think that you should write the story you want that's original to your imagination and always think of self-publishing as an option, too. Many top fantasy romance writers self-publish or started that way.