Select preferred "Watch Now" services
Reelgood, Trakt's (primary?) "Watch Now" data partner, could possibly provide this information somehow with an update to the existing two-way sync feature, if implementing Trakt's own UI for it isn't feasible due to available dev time etc.
The main use cases I see are filtering lists and other views to show only content the user has access to through a service subscription, and prioritizing which two "Watch Now" buttons get the prime slot below the poster image on pages where that applies. (Right now, even shows that are available on e.g. Hulu display Amazon & iTunes purchase options there, forcing the user to open the "Watch Now" modal to access the streaming option they already pay for.)
See https://support.trakt.tv/forums/188762-general/suggestions/17409505-filter-based-on-watch-now-services-that-i-subscr for a related idea (the first use case).