I'm generally not a fan of fellowships in FM. You lose quite a bit of money by taking another year to train, and unlike the major specialties like cards, pulm, endo, GI, neuro, etc, you don't really make much more money when you're done. You also aren't a true "specialist". You're just a generalist with a deeper fund of knowledge in one area.
I recently worked with a FP who has a sports med focused clinic. He's kinda the sports med guy in town. I spent the week with him, and guess what I saw....lots of 65 year olds with cholesterol problems, well-child visits and URI's. In other words, he does family medicine 85% of the time, and then occasionally, he checks out someone's knee or ankle. I guess he has a "horse week" where, for reasons that mystify everyone but 'butterfly effect' people, he has dozens of people in who fell off their horse. One week a year. Never knows which week it'll be.
So, if you want to sub-specialize, don't go FM. Go IM.