After the ending of Avengers: Infinity War shook fans to the core, theories upon theories for the upcoming follow-up have run rampant, especially regarding the fourth Avengers installment's title.
When it was recently announced that it will be called Avengers: Endgame, many instantly found its roots in a quote from Doctor Strange in Infinity War when he hands over the Time Stone to Thanos to save Tony's life and declares "we're in endgame now."
Hmm... makes you think that maybe there's a larger plan at play here, doesn't it? In the clip, Robert Downey Jr. is referring to the plot of the second Avengers flick Age of Ultron, in which Tony and Bruce secretly work on a global defense program called "Ultron" using the artificial intelligence in the scepter containing the Mind Stone. The "Ultron" project of course goes terribly wrong when it becomes the main villain of the film.
These words said by Robert Downey Jr. are also in reference to something his character says in Age of Ultron in a scene where the Avengers learn of Ultron's existence and start pointing fingers at Tony and Bruce for messing with the technology. Tony defends himself by explaining that they were trying to create something that could stand against the alien threats that have tried them before. In Tony Stark's words:
Anybody remember when I carried a nuke through a wormhole, saved New York, recall that? A hostile alien army came charging through a hole in space, we're standing 300 feet below it. We're the Avengers, we can bust arms dealers all the live long day, but that up there, that's the endgame. How are you guys planning on beating that?
And then Captain America chimes in and says "together" to mush up the scene a bit and the Avengers go out to fight Ultron. While it could be purely a coincidence that the word "endgame" was said prior to Infinity War too, it could also be some sort of hint into where the fourth Avengers film is going.
As referenced in both RDJ's words and his lines in Age of Ultron, Tony Stark knows and is fearful of a more aggressive threat than the one that opened a wormhole in New York City in the 2012 film, and he doesn't know how they can be prepared. He tried to create something that can replace the heroes so they don't have to come against it, but it too fails, leaving things to the Avengers.
In the Age of Ultron conversation, Cap and Tony go head-to-head in disagreement as Captain America asserts that it doesn't matter what happens, as long as they fight, and maybe die together, that's enough.
All will be revealed when Avengers: Endgame comes to theaters on April 26, 2019.