It looks like you are definitely on the right track and will be a good candidate for MD/PhD programs. I can't think of anything else you need to do. In addition to the good grades and MCAT score, the admissions committee just wants to see that you are commited to a career in both medicine and research.
It sounds like you have already had a good research experience, so I would say to try another lab if you want. It won't take away from your application. It also may help you to explore another field of science and learn different techniques. And, there is nothing to say that you have to do your PhD in the same field that you did your undergrad research. Just have fun and enjoy it.