Batman and Flash work together to build a Cosmic Treadmill on a Batmobile like in that LEGO Brainiac movie, while Superman flies around the earth really fast to pull a Superman the Movie, and between these three heroes they set the timeline to something that undoes all Snyder did. You could even work out a deal with Fox and/or Disney for a mid-credits scene where Deadpool shoots Snyder in the head looking over a script for Man of Steel.