So many writers fall into the trap of trying to write in the trendy genres and subjects of the day. These people see the successes of authors like Rowling, King, and Grisham and try to emulate that instead of developing their own style. They seek external opinions regularly and always want to know what books people are buying.
The problem is that there is no way to predict what will be the hot literary fad in advance. The publishing process is slow. Many authors can spend years trying to land a deal with a publisher, and even then it can be another year before their book hits store shelves. The subjects that people want to read about today may be dated before the book hits the press.
If you are interested in selling your work, the best agents and publishing houses are not looking for more of the same work that is already out there. They are looking for the next trend and the next popular author.
The best subject for a novel is one that the author is passionate about. What do you love? What do you hate? What questions intrigue you? Ask yourself "what if" questions about situations in your life. People watch. Keep a notebook specifically for jotting down ideas as you think of them.
Walking into any bookstore should tell you that there are a wide variety of readers waiting for new books. Just because certain authors are popular does not mean that other genres stop selling. There will always be people looking for the new and the different.
If you write something that you are uninterested in, your story will probably lack passion. That is, if you even finish it! Writing a novel is hard work, and you would be doing yourself a disservice to try and write 80,000 words on a subject that bores you. There are so many people who want to write a novel, but never get past the first ten pages. You increase your odds of writing a successful draft by writing what you love.
Don't spend time worrying about what has and hasn't been done already. Many of the best books out there are based on ideas that were already written. What is important is your fresh perspective and unique voice.
Write for yourself first. The best subject for your novel is the one that you feel inspired to write about. It is said that everyone has a story to tell - find yours and share it with the world!