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.<p>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.