You can do regenerative medicine research in just about any field of medicine...so find what you like that fits you the best. Doing any sort of regen med project during med school is great too, and nice because you can apply the same principles of biology to a variety of diseases across many different specialties....meaning your research will be valued no matter what you choose to apply for residency.
If you're interested in more of a research career than a clinical focused career, you might want to consider Pathology. Very flexible residency when it comes to integrating research into your time (considering you don't really see patients).