Before I begin, I must warn you that the following contains some potential spoilers for Suicide Squad. Proceed with caution.
It was recently reported that Suicide Squad will feature a somewhat unexpected cameo by Ezra Miller who plays the character of Barry Allen/The Flash in the DCEU. According to Cinema Blend, the cameo is part of a scene where Captain Boomerang is caught by The Flash. During a press interview for the film, Jai Courtney confirmed that Ezra Miller makes a short cameo. Surprisingly, Courtney had no prior knowledge of the cameo and was caught off-guard when he saw the film for the first time.
"That was, whether it was something they knew or not, it was never mentioned [to me]. And so I am probably of the belief that was all by design in order to f*******, you know, just keep it [secret]. I don't know how it got leaked, but I would have assumed they wouldn't have dropped that gem until, you know, they would have just let people see the movie."
The Speed-Force and Time Travel
If Ezra Miller's cameo as The Flash is simply a small Easter egg to showcase the character fighting crime, it would still be a nice addition that fans would enjoy. However, if his appearance has to do with the speed-force time travel, then it could have major implications for future story arcs regarding the character and/or the wider universe. It is well known that unlike most characters of DC, The Flash is able to travel through time and dimensions. We saw a tease of that during Bruce Wayne's dream sequence in Batman v Superman: Dawn of Justice when a future version of The Flash appears to him. If The Flash is indeed time-traveling in Suicide Squad, it could be to set up future story arcs.
Affecting Other Characters
As of right now, we don't know when Suicide Squad takes place within the current DCEU timeline. If the film takes place just after the events of Batman v Superman, then it could be just as likely that The Flash appears in his normal timeline neither from the future or past. However, if the film takes place before Batman v Superman or Man of Steel, this leaves open the possibility that The Flash could appear from the future through the speed-force. This would create all sorts of story possibilities for The Flash to appear in other films through the speed-force that could potentially affect other characters in the DCEU or directly affect his own standalone film. One example of this is the Flashpoint Paradox story arc where The Flash alters his past which creates drastic changes for characters across the DC universe.
At this point, all we can do is speculate about what the appearance of The Flash in Suicide Squad really means, but on the bright side, we only have to wait one more week to find out!
**Update** Early screenings have confirmed that Suicide Squad takes place after the events of Batman v Superman.
Suicide Squad is set for release on August 5 in the U.S.