While I cannot tell you what niche would be best for you personally, I would recommend making something impressive in the process of learning whatever it is you choose. At least then, if the area of expertise ends up being a bad bet, you would have something to show for it; and this portfolio item could live far longer than the specific knowledge you acquire. Another option would be to make the learning process a teaching experience (if you are not worried about sharing trade secrets), such as by writing educational blog posts in the process.
The sense of your question is very much akin to asking for investment advice. Both have the same problems of being a gamble and being prone to having recipients unwilling to divulge their secrets. If I were to provide one piece of advice on choosing, I would say choose a technology related to a field you would like to learn more about for personal, less technical reasons. That way you enjoy the learning and have a more certain feeling of accomplishment.