Standard completion of Arkham Knight is accomplished by playing through the game’s story missions. This completes the tale of this ugly Halloween in Gotham, but that isn’t all the story available. To unlock the full ending, complete at least seven side missions in addition to the game’s story missions. You can accomplish this without focusing on Riddler collectibles: the trophies, riddles, and puzzles Edward Nigma has rigged up all over Gotham. But that’s still not quite all.
To unlock a slightly longer version of the full ending, complete all Most Wanted side missions in addition to the story. Since this includes the “Riddler’s Revenge” side mission, this inherently includes gathering all trophies and collectibles. Completing side missions and tracking down Riddler trophies you’ve missed can be accomplished after completing the main story, so you don’t have to worry about a point of no return for the “full” ending.
Completing missions and snagging collectibles also unlocks Showcase models, Concept Art, Bios, Gotham City Stories, AR Challenges, and (depending on your gaming platform) Achievements or Trophies.