Custom Filters / Tags
I think it would be cool if you could assign your own filters to lists.
For example you could create a tag named "Again and Again" in which you could tag all movies that you can watch all the time.
For me, I would tag all movies that are available on Netflix Germany and would be able to filter my watchlist by this.
Another approach would be to filter Lists by other Lists so that you can see which movies are in both lists.
There would be so many useful things you could do with this.