In addition to Joe Russo explaining why no Hulk for Infinity War and why the Black Order was weaker, James Gunn also explains the ending of Guardians of the Galaxy 2 and how it relates to The Avengers: Infinity War.
The end of Gunn's Guardians 2 saw Nebula going off on her own to try and kill Thanos, which played out in The Avengers: Infinity War; however, Gunn says the Nebula story in Guardians of the Galaxy 2 wasn't because of Infinity War, it all just played out natural.
"I didn't ever have an ending for what Guardians 2 was supposed to be," Gunn explained in the Infinity War Director's Commentary bonus feature. "I didn't have a single story point that was ever discussed with Marvel before I wrote the story. So where that movie ends up, with Nebula going off and doing her thing, that's what I mean by, people, we kind of do our own little thing. We write our own little story and then it goes off to [the Russos]. And I gave these guys a script very, very early on. And they were, like, 'That's perfect, because that works great for us with where Nebula is, what we want to do with the story.' And it goes off."
So we see Gunn reveals that the other Marvel movies and Infinity War didn't influence his Guardians of the Galaxy 2 movie, but it was vice versa, as the Russo brothers used Gunn's idea and incorporated it into Gamora and Nebula's story in Infinity War rather nicely.
Not much is known about The Avengers 4, but Infinity War did see Gamora sacrificed for the Soul Gem - with the Russo brothers having also confirmed that Gamora is within the Soul Gem - and leaked art does feature Nebula, so we see the sisters' story is far from over.
"She in fact is, yes," Joe Russo said about Gamora being trapped in the Soul Stone. "It was an attempt on our part - because we don’t like two-dimensional roles or three-dimensional villains every villain is a hero in their own story and as insane and psychotic and brutal and violent as Thanos is he’s a more complex villain if you go on a journey with him emotionally. He does care for things and it is complicated for him to execute his plan and it cost him something. He said at the end that it cost him everything, and that it was the only thing he loved which was Gamora which is why we put him back with her at the end."
The Avengers: Infinity War becomes available on Blu-Ray on August 14; The Avengers 4 has a May 3, 2019 release date.