Your idea sounds interesting. Recommendation system has been a hot research topic. I am sure you are able to find handy materials from the Internet.
There are several options for the "something extra" part. What you proposed is to do recommendation based on users' watching history. You may try doing recommendation based on the themes/features of films. You may try different algorithms to find the nearest item(s) of a given item. Feel free to change the "something extra" part if you get stuck. Good luck!