The first shot of the opening title is a direct reference to the opening shot of the theme from the 2001 Justice League series.
This movie was foreshadowed at the end of Attack Of The Legion Of Doom (2015), when Darkseid recruited Brainiac.
During the quick fight between Batman and Vandal Savage in prehistory, a few notes from the 'fight scene' music from Star Trek The Original Series is played.
When Batman and Brainiac Superman are fighting, Batman uses the same battle suit from Attack Of The Legion Of Doom (2015) to fight Gorilla Grodd.
The 6 main members of the Justice League in the movie are the same 6 that were in Attack Of The Legion Of Doom (2015). Batman, Superman, Wonder Woman, Cyborg, The Flash, And Green Lantern.