We’ve assumed for the past couple of years that Thanos would get his big hands on the Infinity Stones by the time the Infinity War arrived. However, there’s just one movie left until the third Avengers movie hits the theaters (leaving aside Thor: Ragnarok, which is already a hit in other markets), and the Mad Titan hasn't achieved a thing.
Thanos does have the Infinity Gauntlet, but it’s worthless without the gems. So he’ll personally go after the Stones in Infinity War (Part One, as Mark Ruffalo revealed at 6:53 in the clip below).
During a recent interview with CNET, director Anthony Russo asserted that Avengers: Infinity War is in part a heist movie, and added that the villain will be more successful in his quest for the Infinity Stones:
With Infinity War, the biggest new element to the movie is Thanos and the fact that he's entering the storytelling in a very bold, strong way, to the degree that he's almost one of the leads. We've shaped an interesting narrative around him that in some ways leans heavily on a heist film in the fact that he's going after the Infinity Stones in a much bolder, successful way than he has in the past. The entire movie has that energy of the bad guy being one step ahead of the heroes. We looked at a lot of movies that had that heist-style energy to them, [and] that brought some inspiration.
Of course, we can’t expect Avengers: Infinity War to be like Ant-Man. However, we do understand that a lot of planning is necessary for the villain to stay ahead of the Avengers (and the Guardians of the Galaxy, we assume).
Avengers: Infinity War will be released on May 4, 2018.