According to fan theories as mentioned in Fansided, this makes sense as, "Infinity War is nearly guaranteed to have both a mid-credits scene and a post-credits scene, so even if one of them needs to set something up for Avengers 4 or another MCU film, there's another scene potentially in play".
Entertainment Weekly's bevy of coverage about the movie this week gave us a glimpse at a moment from this lavish fight that we've only seen described to us, as part of closed-door airings of Infinity War footage at D23 and San Diego Comic-Con: A shot of Thanos using the might of the Infinity Gauntlet to literally fling a moon at Earth's mightiest heroes.
Directed by Joe and Anthony Russo, Avengers: Infinity War stars Robert Downey Jr., Chris Evans, Chris Hemsworth, Mark Ruffalo, Jeremy Renner, Scarlett Johansson, Anthony Mackie, Paul Rudd, Elizabeth Olsen, Tom Holland, Benedict Cumberbatch, Chadwick Boseman, Chris Pratt, Zoe Saldana, Dave Bautista, Bradley Cooper, Vin Diesel, Tom Hiddleston, and Josh Brolin. Dating back to the Avengers post-credits scene where he first made his debut, prompting casual moviegoers across to globe to go, "Who's that purple guy?", the big bad has been seen pulling the strings throughout the Marvel universe. They're all Titans and they all look similar, but he was born deformed. You think of overpopulation and killing half the universe in order to save the other half and all this kind of stuff. Jackson, who plays Nick Fury in the Marvel Cinematic Universe, has posted the photo of him getting a lifecast. When you see the relationship with Gamora and you see that evolves, God I wish I could tell you. "And what I see is this in this guy's eyes - this super, super, super intelligence". "He stuck out. He was anomaly. And that lent to this apparent insanity". "There's this constant contrasting thing about this Neanderthalic lughead who's way more intelligent than anybody else in the movie, by far". I think we're seeing it shift from a very purposeful initiative to just a fact of life, to just a way of doing business. "But he has a capacity to love very much and very deeply".
From Brolin's description, it sounds like Avengers: Infinity War will swing for the fences to make Thanos engaging and three-dimensional.