Guesses are being made that the Knightmare sequence in Batman v Superman is actually takes place after the newly revealed Justice League Knightmare sequence, ruling out the possibility of it all being a dream.
Theories about those Knightmare sequences lean in on Joker being the center point of the connection between BvS and Zack Snyder’s Justice League.
Snyder has long thought of Batman and Joker’s role within DCEU and how the Knightmare connection between the two, which he began to set up in Batman v Superman: Dawn of Justice.
The gun Batman carries in Batman v Superman is Joker’s gun, which we saw him holding in the Snyder Cut trailer.