Part of Justice League's original ending from Zack Snyder's version has been (possibly) revealed. Warner Bros. handed the keys to the DC kingdom over to Snyder when it was decided that Man of Steel would be the start of something greater. He went on to direct Batman v Superman: Dawn of Justice, but the divisive response changed what else Snyder had in mind. Instead of a five-film DCEU story where Justice League was the third chapter, it was tinkered with tremendously - especially after he left.
One of the rumors that arose after Joss Whedon took over Justice League to oversee reshoots was that the ending of Snyder's story was being overhauled. Based on reports that Darkseid was going to be set up as the big bad of Justice League 2, it appeared Whedon and WB were scrapping any Darkseid set up.
A fan on Vero posted a still from the Justice League special features that showed the team fully assembled and looking up. Based on the amount of green screen and the set they are standing on, it is clear this is from the third act of the movie. The fan asked to see if Snyder could comment on what was being shown and Snyder did just that. In doing so, Snyder revealed that the image below is from Justice League's finale - and that they were staring up through a boom tube.
The theatrical cut of Justice League did end with Steppenwolf and the Parademons attacking him being taken through a boom tube, so this is not confirmed to be part of the Snyder Cut. This could instead be an alternate take on the reaction shot of the team seeing Steppenwolf being taken. However, there are a few differences between this image and the theatrical scene. The team is incredibly close together in the photo, but are scattered in the theatrical cut with each getting single reaction shots instead of a group one. Additionally, Ezra Miller's Flash is laying on the ground in the theatrical cut when Steppenwolf is taken, and not standing behind the team.
While there is a possibility that this was simply an alternate take, it lines up much better as being part of Snyder's original vision for an epic ending. It's known a young Darkseid was supposed to appear in Justice League so they could further set up his true introduction at the end. Darkseid bringing Steppenwolf back to Apokolopis to answer for his failure and then set up the powerful DC villain to take on the League themselves in a sequel would make for a giant cliffhanger that would've also introduced the heroes to their next foe. Batman even had a line that was referencing Darkseid's arrival, so this is clearly where Snyder was going. Unfortunately, this tease may be the closest fans will come to seeing Snyder's Justice League ending.
Source: Zack Snyder