Being able to view the collection that a movie may belong to would be nice to see. Not just is this for general viewing pleasure, but it also will help fans in discovery. If say I really liked a movie and was unaware there was a sequel, this would be a nice thing to have for this purpose.
I believe though that Collections are only avail via the TMDB API, and I cannot speak to how good or reliable this data actually is. I understand the current design... where the Trakt data is 100 reliant on TMDB (and others) data... but I think that Trakt could start to offer more unique services and features if the Trakt data migrated to be based off of TMDB data (and updated), but also linked to a Trakt data subset... Yes, dangerous road to go down that needs careful consideration.