I think a very useful feature would be the ability to add "tags" to each entry to make searching easier. For instance, being able to add tags such as "Russian" or "silent" or "samurai" would make the search function in XMM both easier and more powerful when trying to find a film of a certain type, or perhaps can't remember the title.
The filter function is good for a basic search, but tags are useful for finding, for example, a film you might not remember the title of, or if you're looking for a certain kind of movie. Tags would enable a more advanced type of search.