Although we suspect that the Infinity War won't end with next year's Avengers (with its expansive storyline), we aren’t completely certain what will come next. We don’t even have the title of the fourth Avengers flick. This remains one of the Marvel Cinematic Universe’s biggest and most closely guarded secrets ever.
Some time ago, Kevin Feige said that revealing the title of the (presumed) final Avengers movie would spoil the outcome of Avengers: Infinity War (previously, the two Avengers movies were just called Part 1 and Part 2). The sequel could have gone in any direction, so we actually believed that. Mark Ruffalo recently fueled further speculation when he said that the studio plans to develop a Hulk trilogy that will begin with Thor: Ragnarok and end with Avengers 4 (World War Hulk, anyone?).
During the Ragnarok press junket, Screen Rant asked Feige about the mystery surrounding the final Avengers flick. Feige responded that the fans have blown the secrecy out of proportion. He added that the title hasn't been revealed because the studio wants to keep the focus on Infinity War.
Yeah, I think some of that has been blown out of proportion. We do want the focus to be on Infinity War for now.
So it all comes down to marketing strategy. Revealing a movie’s title – or not revealing a movie’s title – can be considered part of the marketing campaign.
From that point of view, it makes sense. The fans are constantly talking about upcoming Marvel Cinematic Universe projects, and that certainly holds true for Ragnarok, Black Panther, and Infinity War. For the moment, it might be advantageous to steer the conversation toward those movies.
The movie’s title may be a spoiler, but that isn't the only reason (or the biggest, as Feige said) for keeping it under wraps.
Avengers: Infinity War will be released on May 4, 2018. Avengers 4 will follow on May 3, 2019.