I went from Bujinkan to BJJ. I was in a really crappy shape when I started bujinkan. While doing it I lost about 30 pounds, although mainly from my strict diet. But Bujinkan did help me get started. I would have been killed if I had gone straight to BJJ.
So if you feel your school might not be challenging you enough anymore I heartily recommend at least trying out BJJ. It's a grueling, punishing and insanely stupid hobby (although I just might be injure prone) but I love it for fitness and for what it's teaching me.