I've recently begun my first try at querying a novel. Been doing it slowly and learning a lot along the way. I know the sage advice is to immediately begin writing the next book, but I first had to catch up on work-related education that I had put off while finishing my novel. Now that's done, and I should be writing, yet I can't decide what to write next.

Ideally, I'd want to stay in my ancient Sparta and "build a brand", so to speak, though I don't plan on an actual series at this point. I love writing in that time/place. But I'm afraid to spend so much time on a new book set in the same time/culture of my current one, if this one doesn't garner me an agent. (Writing historical fiction takes me a long time, because of all the research involved.) The agents who have been interested off the query (4/11 queried) are all from top agencies, so I at least know the concept seems marketable to them or they wouldn't have requested pages. Another part of me is tempted to write a completely different sort of book (contemporary romance with a bit of magic). That would be far from my ancient historical women's fiction which has a more serious tone, and it might not even be appealing to whomever might want to rep my current story, if that happens. But who knows - that sort of story could be my niche.

Any advice?