Yes, I'm aware we're not supposed to say "it would be so easy", but I'm going to anyway. It adds nothing new to the core of the site, and uses nothing new in the database. Unless your DB makes no sense at all (given how well the site runs, I'm willing to bet it's not), this feature is roughly 98% copy and paste from the existing recommendations part--just have to change the WHERE clause of the recommendations selection from picking the single id to picking everything IN a subquery of that user's watched list. Bonus points if that subquery is limited by the user's rating, but it's not necessary.
just sayin, i pretty much immediately have no interest in answering someone when they take the time to tell me how 'easy' something is. i'm not giving any new information though given the line in bold.