Tags seem like a good idea, the only thing that worries me is that they would be easy to abuse. That being said, I'd be willing to bet it would come out better than what we currently have.
However, if I made a new video platform (Daunting task) I would penalize content creator who purposefully mis-categories their content.
That should cover your frontend, so you could focus on the recommendation scheme.
Edit: If you do decide to play around with it and decide to open source it, could you send me a link to your repo? I'd be interested to watch, and possibly contribute here and there. My email is in my bio.