Avengers: Infinity War did an impressive balancing act when it came to giving the massive number of characters enough time on screen. The villain Thanos got more than most, but apparently, there might be even more on the way. Thanos creator Jim Starlin recently appeared at the Amazing Comic-Con in Las Vegas and told the assembled crowd that there was as much as 30 minutes of Thanos backstory that was shot and completed, but cut from the final film in order to keep the runtime down to something vaguely acceptable. According to Starlin, the plan is to include this footage in a future Blu-ray release of Avengers: Infinity War.
Avengers: Infinity War was already over two and a half hours long, so it's not shocking if they decided to cut a lot of backstory for Thanos that wasn't strictly necessary. Thanos explains to Doctor Strange why he decided to begin his quest for the Infinity Stones, and we see how he "adopted" Gamora. Those are really the only really important pieces of Thanos' history that the movie needed to give us.
However, Collider reports that Jim Starlin said that while visiting the set of Avengers: Infinity War the writers and directors spent a lot of time asking him about Thanos and really tried to dig into his history. However, he was informed shortly before the film's release that a large chunk of Thanos' backstory that was filmed was being cut out. Starlin's current understanding is that this material will go on some future Blu-ray. It's not clear if the scenes will be re-edited into the film as an Extended Edition or if they'll simply be included as extras.
The Russo Brothers have already said that the Blu-Ray for Avengers: Infinity War will likely include a lot of deleted scenes, as they encouraged a lot of improvisation on set, and thus have many different takes of the different scenes in the film. However, these scenes would be something else. While one assumes some of these scenes would give Josh Brolin even more time to shine as Thanos, some of the sequences apparently included seeing Thanos as a youth, so we apparently would have seen the character grow up on Titan and learn about all the key moments in his life that led to his decision to find the Infinity Stones by watching him grow up rather than simply watching explain it all to Doctor Strange.
As this is second-hand information, it's possible that the game of telephone has resulted in some of this detail getting confused, but if there is really 30 more minutes of Thanos material from Avengers: Infinity War that's been completed, certainly the fans of the movie are going to want to see it. We'll keep our eyes open for the Extended Edition Blu-ray that we hope is really on the way.