[SPOILERS for Ant-Man ahead.]
Captain America: Civil War - the first installment in Phase III of the Marvel Cinematic Universe - naturally had a major presence at the 2015 D23 Expo; there, among other things, a sizzle reel for the film was shown, teasing the schism that forms within the superhero community in the movie and gives rise to said 'Civil War'. Meanwhile, Steve Rogers/Captain America (Chris Evans) also carries out a more personal mission in the movie: find Bucky Barnes/Winter Soldier (Sebastian Stan).
Ant-Man's post credit scene has already shown that Steve and Sam Wilson/Falcon (Anthony Mackie) do, in fact, catch up with Bucky in Civil War, in addition to foreshadowing that Scott Lang/Ant-Man 2.0 (Paul Rudd) will be their ally when trouble comes knocking. Previously-leaked photos from the Civil War set revealed Bucky crossing paths with T'Challa/Black Panther (who, in the MCU, is going to be played by Chadwick Boseman), while the latest Civil War set pic shows Bucky and Sam in what one presumes is a dangerous situation.
The latest Civil War set photos come from the production's ongoing shoot in Germany, where new Peter Parker actor Tom Holland recently traveled to shoot additional scenes (as the MCU version of Spider-Man) for Civil War directors Anthony and Joe Russo. It's not yet been revealed which side of the conflict in the film - with Steve and Tony Stark/Iron Man (Robert Downey Jr.) leading the opposing factions - Spider-Man falls on.
However, the new set pic posted by German site Bild.de (hat tip to CBM) shows that Bucky and Sam will be able to set aside whatever differences they have long enough to take a run together... Or maybe they're just having a friendly footrace. You can check out said Civil War set photo, below:
— BILD (@BILD) August 18, 2015
Civil War - adapted from the Mark Millar and Steve McNiven comic book (of the same name) and written by Christopher Markus and Stephen McFeely (Captain America 1 & 2) - examines what happens in the MCU when a government clamp-down on superhero activity leads divides the costume-wearing community. Not every superhero has a strict allegiance; Black Panther, as an example, is rumored to be carrying out a mission of his own in the film (one that involves Bucky, no less), and Marvel President of Production Kevin Feige has also confirmed that T'Challa is an impartial player during the 'Civil War'.
In a D23 interview with EW, Evans noted that elements like this - where characters struggle to pick sides (if they choose one at all) - helps to set Civil War apart from previous Marvel Studios movies:
“A majority of the Marvel films, it’s clear to know who’s right and who’s wrong. This is one of the first movies where it’s parallel to the arguments, I think, we have in life where nobody’s right, nobody’s wrong. It’s just a matter of opinion.”
That's not to say Black Panther will, per se, see himself as Steve and Sam's enemy because of their ties to Bucky in Civil War. Evans and Mackie have already clarified that both questions that involve who is right and who is wrong and who's on whose side in the film aren't always the easiest ones to answer. Likewise, due to his long history with Bucky (and his hope that his old friend can still be saved), Steve isn't exactly the most impartial party when it comes to issues concerning Winter Soldier and whether or not he's an ally that can be counted on... and not even Sam believes Steve's faith isn't misplaced.
In short: even as civil war erupts, the Steve/Bucky plot thread should be as pivotal (even more so) in the third Captain America movie as it was in the Russos' previous installment, Captain America: The Winter Soldier.
Ant-Man is currently in theaters; Captain America: Civil War opens on May 6, 2016; Doctor Strange– November 4, 2016; Guardians of the Galaxy Vol. 2 – May 5, 2017; Spider-Man reboot – July 28, 2017; Thor: Ragnarok – November 3, 2017; The Avengers: Infinity War Part 1 – May 4, 2018; Black Panther – July 6, 2018; Captain Marvel – November 2, 2018; The Avengers: Infinity War Part 2 – May 3, 2019; and Inhumans – July 12, 2019.