Getting 100% in Batman: Arkham Knight and unlocking the 'true ending' doesn't necessarily mean seeing everything Arkham Knight has to offer. It's actually pretty easy to miss something within said true ending. (Spoilers to follow!)
Late last year, Batman: Arkham City made headlines after fans found an easter egg a whole three years after the release of the game. Turns out, if you set your machine's date to December 13, 2004, a character called Calendar Man will have new dialogue. You can view the scene in full here, but the pertinent stuff is when he says "The end of days is coming. I was there at your beginning, and I will be there at your end."
At the time, it seemed like typical villain mumbo jumbo. But, funnily enough, Calendar Man did indeed keep his promise in Arkham Knight.
Here's the true ending to Batman: Arkham Knight, courtesy of RabidRetrospectGames:
Don't see what I'm talking about? Watch around the 1:50 mark. You should see this:
That guy to the right, there? That's totally Calendar Man, and he's present during Bruce Wayne's "death." For reference, here's what he looks like in Arkham City:
Picture: Batman Arkham Videos
Same head tattoo/branding stuff. Same guy! I'm impressed that Rocksteady included such a small detail. I guess Calendar Man is a man of his word.