My favorite aspect about mega franchises: When they turn minor details into a major plot-point for a different series, installment, spin-off, or something else entirely.
For example, the backstory about how the Rebels stole the Death Star plans was glanced over in “A New Hope” but became the main focus of “Rogue One”.
Another example, “Daredevil” took a biker gang that appeared in one, filler-ish episode of “Agents of SHIELD” and turned them into a major, villainous faction in season two.
Then there’s the Walking Dead web shows that focused on very minor, unimportant details in the main show. The first one focused on the bike zombie in the first episode, the second one focused on Rick’s photo albums, and the third one showed the origin of the “Don’t Open, Dead Inside” message.
What about the rest of you, what’s your favorite example of a minor detail turned into a major plot device for some other series.