There are massive SPOILERS for Avengers: Infinity War below, so read ahead at your own risk. The character of Nebula has gone through quite the story arc since her Marvel Cinematic Universe debut in Guardians of the Galaxy Vol. 1. She goes through quite a bit of torture in Infinity War, only to end up helping members of the Guardians, Spider-Man, Iron Man, and Doctor Strange on Titan as they battle Thanos. However, a new theory suggests that she could very well end up as the villain in Avengers 4.
In the comics, Nebula ends up enduring a lot more torture, which ends up turning her into a zombie-like character. After Thanos snaps his fingers and completes his goal, he teleports to another planet and slowly starts to doubt his actions, which is when Nebula is able to snatch the Infinity Gauntlet from the Mad Titan, becoming an all power being who is not in her right mind, due to all that she has been through. The storyline involves Thanos joining the Avengers to get to the Infinity Gauntlet and place it and the Stones in their rightful places.
This is quite a stretch for Avengers 4, but then again the screenwriters and the Russo Brothers have hinted that the upcoming movie will be nothing like MCU fans are expecting. Additionally, Infinity War took from different pieces of source material and put them together, editing it to make it work for the narrative. For instance, Adam Warlock is not in the movie, and James Gunn has said that he is not in Avengers 4 either, which put Doctor Strange in his position. In the comics, Adam Warlock receives the Gauntlet and distributes the Stones accordingly.
That being said, we could certainly see a play off of the original source material in which Nebula is the one to retrieve the Infinity Gauntlet from her father. She might not be the villain, but you never know what all of that power could do to someone who has been tortured their entire life and just lost the only person that they truly cared about. It could make for some pretty intense scenes in Avengers 4, which is being teased as an even bigger ride than Infinity War.
Stephen McFeely and Christopher Markus have cherry picked source material from the comics, so it will be very interesting to see what they took for Avengers 4, since it was written at the same time as Infinity War. Nebula could very well end up as a villain for the portion of the movie, or she could be the one to finally kill Thanos. Karen Gillan has teased that her Nebula character has a pretty large part to play in Avengers 4, which makes sense since she barely had any screen time in Avengers 3. This theory was first reported by ComicBook.