More user settings on category's and allow/disallow to play the media. Category view access but not allow to enter rights. It's now very basic to control users on specific category. Integrate rights like XF allready have!
I'm not sure how possible it would be to prevent users from playing much of the media, at least that which comes from other sources (YouTube, Vimeo etc) as it's not actually hosted in the Gallery. Even if it was possible to do this, users would soon wise up and visit this media directly on YouTube/Vimeo etc, thus driving away traffic from your site.
That might be a good point you have. Controlling users(groups) in the gallery categories can be more specific with more rights. So they can see certain catergories but can't access it, this can tricker to do more for the community to earn xs to it.