If you watch Justice League back now and think "what this film needs is a massive God on God battle", then boy, are you going to be PISSED.
That's exactly what Zack Snyder wanted to include in his version of the movie, but his fight between Ares and Darkseid - which was actually filmed - didn't make the final cit. The studio clearly decided it was part of the things that they simply didn't enjoy about his creative vision for the DCEU.
Darkseid was initially intended to be part of a flashback sequence that would have seen his first attempt to take over the Earth and there was supposed to be a battle between the leader of Apokolips and Wonder Woman's nemesis. That's why David Thewlis was created as playing Ares in Justice League despite the fact that you can barely see the character.
In reality, it's not even him playing the character. Actor Nick McKinless was hired to play Ares for the flashback and shot the fight sequence after preparing for the role for four months. Hence those ridiculous abs that were shared by DC Last Stand on Instagram...
McKinless responded to that post saying:
"This is correct. I played Ares in Justice League (not the previous actor). I was NOT credited for my work. We shot a whole fight sequence with Darkseid. I trained for 4 months to get in that shape. Zack was amazing to work with. Thanks for posting this ."
Interestingly, he was asked (repeatedly) about the scene and whether he would be willing to talk about it. He seems keen (though who knows what his contract situation is), but he won't until he has the right opportunity:
"I would like to talk about it more but want the best possible platform to highlight our work and where the history lesson could have gone. Maybe we should make a fan made film focussing in the history lesson..."
That's another detail for the Snyder Cut detailed and yet more ammunition for the fans who want to see it. You have to wonder how much attention Warner Bros give all of this...