Sports cardiology is a very interesting and cool niche. Fellowships generally only exist at very large (and "name brand") institutions, which may be a factor in pursuing the fellowship (i.e. may improve your marketability and make you more appealing on the job market). Having a niche is never a bad thing, but it means you will have to work to raise awareness and build up the practice (almost like docs of yesteryear).
Sports Cards is really cardiology of highly active and athletic people. People who are highly active present in unusual and uncommon ways and treatment differes from general population (e.g. rarely with classical anginal symptoms and may perform revascularization whereas less active people would continue using anti-anginal therapy due to intolerance of traditional medications). Getting into the nitty gritty of CPET as well as nutrition/working closely with other sports medicine (inc ortho) specialists will improve overall marketability and practice building because there are plenty of people who want to prevent disease as well as maximize their cardiac function. Just look what happened to people with Peleton during the pandemic.
