Fake Nationality: American actors playing the Canadian characters. The smattering of British actors (most notably Geoffrey Toone as Dr. Baird) are presumably meant to Brits, though.
The comic book
Christmas Rushed: Not the story itself, but it is the reason the third part of the Knightfall saga, KnightsEnd, incorporated Robin, Legends of the Dark Knight, and Catwoman to tell the story, so the Bat-titles could participate in this story.
Years after Zero Hour had been finished, Alex Ross proposed a limited series entitled Batboy. Based off the Super-Sons imaginary story from the 1970s, Batboy featured an idealistic future of the DCU, with Batboy and his best friend Superman Jr. as the main characters. The original JLA would've been retired, with the Teen Titans, Batwoman (Barbara Gordon) and Superwoman (Kara Zor-El) now comprising the League. Other heroes would be Barry Allen's kids the Tornado Twins, and Aquaman's still-living son, Arthur Jr.. The Joker would still be active, only with a teenage daughter by his side. Harvey Dent would've been cured of his psychosis and acting as a tutor for Bruce Jr., who would have a baby sister named Helena. The only still active member of the original League would've been Hal Jordan. How this ties into Zero Hour is that Batboy would've realized this world seemed too ideal, and would discover that, this was the end result of Hal's attempt to remake the DCU, and would confront him on it. Character designs and plot information for this series were released in Ross's art book Rough Justice.
In the foreword of the 2018 hardcover of Zero Hour, writer/artist Dan Jurgens hinted that the Time Trapper who appeared at the end of the series was the alternate universe Batgirlnote which might explain the long hair and almost feminine-looking face (who sacrificed herself to save Damage), but nothing came of it.
Jurgens also planned to bring back Earth-Two (and by extension, the Pre-Crisis Multiverse) and put the JSA on it.
What Could Have Been: There was a subplot of Yugi obtaining the Millennium Puzzle which would've revealed that Yami was in there and must reunite with Atem, to which Yugi would've learned of the past world. However, this was scrapped since it would've sidetracked from the main story itself.