Like many of other video games presently on the market, Injustice 2 also contains a few Easter Eggs within itself. Unfortunately, these are pretty limited when you consider that the Injustice Series is based upon its own Comic Series, as well as the DC Universe as a whole for each given character. In the end, there are a total of five different Easter Eggs: three are in the form of stage references during gameplay, while the other two have to do with given quotes made by characters while playing.
Stage References
1) The Ace O'Clubs Bar in the setting of Metropolis was first seen in Adventures of Superman #428, being owned by Superman's biggest fan with the name of Bibbo Bibbowski. This same bar is found in the DC Universe Online Game.
2) With the prop room of Gotham near mid-stage is a poster in the back ground that read "RETURN of the PHARAOH". This is actually the same name of an issue in the Danger Trail Comic Series.
3) If a player is to throw an opponent into the theater marquee found in Gotham three times, some of the lettering will fall off and it will then read "Finish Him". This pays homage to the Injustice 2 developers of Mortal Kombat.
Quote References
1) If you have a match between Supergirl and Atrocitus and Supergirl wins the match, she will exclaim "Streaky is better than Dex Starr!" This is her referring to the Super Cat known as Streaky, her very own pet, and comparing him to Atrocitus' companion in Dex Starr.
2) When you begin a match between Batman and Superman, there is a small chance that Batman will say that Superman belong in prison. There is even a small chance that Superman will say he'll rebuild his government. Both are in reference to the events that occurred in Injustice: Gods Among Us.