What's your favorite headcanon?

[Agrippa]

[For Final Fantasy VII and inspired by watching season 3 of Fargo.]
Nibelheim and the general region it's a part of are its world's version of Minnesota. Therefore Cloud Strife and Tifa Lockheart have Minnesotan accents. I'm not sure about everyone else though.

[Mary Poppins/John Wick from my stalled Shadows Over the Golden State crossover.]
While Mary and her flying umbrella can travel rather easily from town to town and city to city in the UK, she still needs to board an airplane like anybody else. So when she ends her Trans-Atlantic flight Miss Poppins, being a rather private person, then departs for the only hotel in the States' Eastern Seaboard where she can find discreet accommodations to her liking, the Continental. Where the two most important rules are "Do not conduct business on Continental grounds" and "Leave Miss Poppins alone, if you value your life and general physical well being." Anyone with at least half a brain approaches Mary Poppins with the same fear and reverential respect typically reserved for John Wick and Winston, that includes John Wick himself. She's not in the same business as the rest of the Continental's clientele, but Mary Poppins' long, magically prolonged life and youth, combined with her magic and her bartitsu skills that come with it, make her the deadliest of it's clientele.