If I work with a publisher, then sales are important. If the book doesn't sell well due to lack of interest, distributors won't stock it, and people won't see it on the shelves. Eventually the book goes out of print. IMO creating a book for a niche audience defeats the purpose of working with a mass market publisher. Better off self-publishing in that situation.
Since this thread started with the idea of doing another book for Hay House, that was the context. However, I have no problem doing an ebook on a topic that may not appeal to a smaller number of people buy which could still be very impactful for its readers.
Right now, however, I'm thinking about what sort of book might be good for mass market distribution.