Marvel Studios President Kevin Feige is the mastermind of the Marvel Cinematic Universe and it's no secret that he hides Easter Eggs in these movies which pay off further down the line (Wakanda, for example, was first teased in Iron Man 2). Now, one popular fan theory doing the rounds points to a seemingly meaningless line in Captain America: Civil War actually tying into next year's Captain Marvel.
When Tony Stark confronts Peter Parker in Aunt May's apartment about him being Spider-Man, the wall-crawler shrugs off the videos he shows him, comparing it to "that video" before Iron Man interrupts and asks, "Oh, you mean like those UFOs over Phoenix?" What you may not realise is that was actually a reference to The Phoenix Lights, a real event which took place over the skies of Mexico in 1997 when thousands of people saw what they claimed was a triangular-shaped UFO lit up.
This fan theory suggests that what those bystanders really saw was the Kree-Skrull War and Captain Marvel (who has been spotted in her red, gold, and blue costume in one official still standing somewhere that looks an awful lot like the desert). This would mean she returned to Earth in 1997 after leaving a couple of years beforehand. If correct, this is an amazing Easter Egg and a great pay off.
We'll just have to wait and see, of course, but we know the Skrulls want to invade Earth so it would make sense that this conflict could happen above Earth only to be written off as a standard UFO event.