Be brutally frank in your answer, because I assure you that the marketplace will be, and by then you may have invested a few thousand dollars to fill your garage with cases of books. Don't ask your partner, your parents, or your children. Forget your friends, too. After all, they want to stay your friends. So, with a bucket of cold water in our faces, let's go on.
Topics come in waves.
A few years ago, you couldn't go wrong with angels, but today publishers want nothing to do with the winged critters. Watch the movies and leading edge TV channels. A recent market survey revealed that the market wanted ancient and extraterrestrial wisdom. For example, I've just ghost-written a book about NDEs, UFOs, ETs, abductions, ancient Egypt, new science, and the Photon Belt. Talk to your local bookstore about what they're ordering nowadays.