Another thing I thought about was maybe having some member-only access to threads--like maybe on the ones where hSDN alumni/undergrads share experiences about college or the "Day in the Life of" doctor threads. This would increase registration since I've noticed that SDN gets many guests online (10+ almost every time I log in) but relatively few members in comparison. It would also be great if you sticky these threads too, ensuring future registrations from high schoolers thinking about a career in medicine.
It would also be wise to include some Vets, researchers, dentists, nurses, PAs, etc. in these "Day in the Life of" threads as well. I'm sure there are some SDN members here willing to do that.
I'm not sure a partnership with CC would be the best. Like INS said above, that site relies on the traffic of kids that just want to get into an Ivy League school or other prestigious school. I've seen many on there want to do I-banking/economics for serious cash. The site is infested with trolls and students bragging about their high school achievements (Example: "Look at me! I have a low 2350 SAT and a horrible 3.9 GPA. Can I get into [insert prestigious school here]?") The long-time posters are mainly parents that have multiple kids. All they do is brag about how they're doing in school.
Moreover, most of the people on CC's med sub-forum have SDN accounts or are parents of pre-meds/med students. There are many high schoolers there that are insanely focused on getting into a BS/MD program, but they're much like their non-medically interested counterparts--braggarts and people obsessed with getting into prestigious schools.
Like you said about TPR, CC also has bad moderation. Every so often trolls appear saying they got accepted somewhere with a low GPA/SAT/ACT before acceptances even came out or saying they have cured cancer or have schools like Duke, Michigan, Berkeley, Northwestern, etc. as a safety.
Quality over quantity, I suppose, is what will distinguish this forum over CC.