Justice League was supposed to be Warner Bros. and DC Films' answer to Marvel Studios' Avengers, but it didn't quite succeed. To be honest, the film seemed somewhat doomed as soon as production troubles starting hitting it, particularly when Zack Snyder had to step down months into filming. Due to personal circumstances, the director of Man of Steel and Batman v Superman: Dawn of Justice had to abandon the third movie in what he saw as a trilogy, leaving Avengers director Joss Whedon to step up and bring it home.
Whedon had already been brought onto the project to help lighten the tone, a move the studio mandated in light of criticisms leveled towards MoS and BvS and the subsequent success found by Wonder Woman. That Whedon had helmed two installments of the franchise Justice League was directly aiming to emulate was an obvious plus; unfortunately, things went downhill from there. Fans and critics alike felt the final cut was a mismatch of tones, resulting in a film marred by two distinct and clashing visions.
Disappointed fans began pining and campaigning for a "Snyder Cut" of the movie -- a version of the film that was solely Snyder's. Fans have signed online petitions and rallied outside of Warner Bros.' studios in hopes that this cut does indeed exist, and will in fact see the light of day. This belief was further fueled by a German Amazon listing that seemingly confirms that an Extended Cut not only exists, but is expected to arrive alongside the movie's home video release.
If Justice League is released with an extra 15 minutes, it wouldn't be the first movie within the DCEU to received an Extended Cut. Two of the other biggest disappointments in the shared universe, Suicide Squad and Batman v Superman, both received a longer run time when they became available on home video, and we're sure the studio noticed that a lot of fans marked these version as improvements on the theatrical cuts. That said, Warner Bros. can't expect to just add in previously-deleted scenes and expect fans to fall in love with the slightly-altered product. Surely, most people won't be interested in watching an even longer version of a film they already actively disliked.
Warner Bros.' only unequivocal DCEU success is with Wonder Woman, a film that reportedly didn't have a single deleted scene of consequence. The studio left the movie's fate in the hands of director Patty Jenkins, mandating neither a secific run time or tone, and it surprised everybody, grossing well over $800 million dollars worldwide. The movie didn't receive an Extended Cut because the majority of what Jenkins filmed was there, in the theatrical release, which audiences were perfectly happy with.