It would be great to be able to apply a filter to your watchlist. Just a drop-down like there is when you are browsing films, to show just horror films on your watchlist, or just comedies.
I'd add that it would be great if we could curate our own categories/tags/notes/whatever-you-want-to-call-it, too! (So not just genre but "comfort" "date night" "recommended by X" etc.)
I want to be able to filter by watchlist by country too... in stats we can see a breakdown of films we've watched by country so it would be cool to see what are all the "African" films in my watchlist for example.
or to be able to filter by actor