If martial arts interests you, start this review blog focussing on reviewing and recommending the best martial arts gear and equipment. There are a lot of disciplines to cover such as karate, taekwondo, Brazilian jiu-jitsu, and Muay Thai etc. There are not a lot of people blogging about these.
Write buying guides, product comparisons, and reviews of training equipment, protective gear, uniforms, and everything else in the game. Additional articles can be on training techniques, benefits of martial arts for self-defense etc. Aim to build a one-stop source of information to enhance training experience. This can be a very profitable blog in the long run.
Blog Name Idea: UltimateMartialArtsGear.com