Ed Boon contributed to the creation of both Injustice and Mortal Kombat. Ed Boon, a member of NetherRealm Studios, is very vocal about what is to come and often teases us about the future. He wrote a tweet that Zack Snyder saw as an encouragement to make a live-action adaptation of Injustice.
Zack Snyder hinted at portraying the story on the silver screen some time ago. It was something that Ed Boon desired; even seeing an animated version of his story made him want to see the live-action version. Ed Boon may hint at the future since he is always known to know more than he admits.
Injustice game director wants Zack Snyder to make a Live-Action version of Injustice
Zack Snyder originally intended for his Justice League to be loosely related to the Injustice series. Justice League showed Superman as tyrannical and even evil, but this was predictable and not how a comic would handle it. Unlike Zack Snyder, who was less likely to get along with DC or longtime comic fans, Injustice always had the support of comic creators because the storyline was good enough to appear in one.
Justice League has given Zack Snyder a chance to see his vision come true, but it's not enough to say that his relationship with DC is over. As opposed to giving up Injustice to Zack Snyder, DC will make an official animated adaptation of the story. Animated movies and TV are things that DC excels at (in contrast to live-action movies), so they don't need outside help to make animated movies.
Zack Snyder seems to have wanted his own take on this universe if Ed Boon is correct. The Director of the Injustice and Mortal Kombat series, Ed Boon, expressed excitement over the upcoming DC animated Injustice adaptation and his admiration for Zack Snyder's adaptation.
It seems unlikely that Zack Snyder will ever be able to revisit this story on the big screen, but it does appear Boon liked Snyder's general vision. In Batman v Superman: Dawn of Justice, Snyder confirmed that the "Knightmare" scene was inspired by Injustice, which is clear to see. NetherRealm captured the dark world better than Zack Snyder, but his version was good enough for Ed Boon to like it.
The fans of Zack Snyder may be hanging on to hope, but DC is very unlikely to hand over the responsibility to Zack Snyder. I'm happy about that. The Injustice team respects the comic books and the characters and doesn't try to change their personality. It's why most of my theories for Injustice 3 revolve around the comics, because NetherRealm respects the source material (and I love comics!)
NetherRealm wanted to keep their characters as close to the source as possible, in contrast to Zack Snyder. According to Zack Snyder, he doesn't think the established lore of the comic books for Batman is realistic, which is why his version of Batman doesn't go over well (even though he looks the MOST like Batman.) WB may give in after an additional petition; after all, money talks (just remember that Flash can outrun Superman any day of the week.)