Simple question: who is causing the dreams/hallucinations that Bruce Wayne experiences throughout the movie? If not a person, then what?
The most popular and likely theory is that this visions are caused by The Flash's time travel.
We see The Flash in that scene, reaching out from a time portal, talking to Bruce about coming back too soon:
Am I too soon?
I'm too soon!
Now, what Bruce is actually experiencing is up for a little bit of debate, but the prevailing theory is that Bruce having his memories of a possible future/alternate timeline (a Flashpoint or a Time Boom) mingle in with his own.
This is a concept that's been explored in Flash storylines before, most famously the Flashpoint Paradox.