If you're ambitious, I think a compelling rap genius style social media site could be built based on people commenting on specific segments of YouTube talks (and podcasts, whatever). Rap genius (now genius.com) seemed to be doing something really novel, but then....something happened. But I think the idea is rich with opportunity.

This project is doing a variation of it:

https://news.ycombinator.com/item?id=31527544

My vision of it would require some pretty strong UI skills, or maybe nowadays it's not so hard to accomplish impressive things.