It was revealed this week, it's possible the real reason between the split of Marvel and Edgar Wright on the Ant-Man production. The likelihood was due the connectivity of the Marvel Cinematic Universe to Edgar's existing script. It's believed that Wright was keen on keeping his characters within his film, along keeping out references and characters from other films. Basically, he wanted to make a Marvel film set-apart from every single Marvel movie since Iron Man. I could see how that would cause massive tension between Edgar and the studio.
I'm unsure if the huge connection between Hank Pym and S.H.I.E.L.D., which is now a key plot point of the film was the main problem or only part of it. It's been confirmed that Agent Carter and Howard Stark will be appearing in the film, to connect Pym to the early days of S.H.I.E.L.D. It's possible he didn't want his film to be a cog in the Avengers machine, which is why they've been kept out of the Avengers films. There's only so long that Marvel could have played that game with Edgar.
We know that this could have been the main reason why both Ant-Man and Wasp were held-back in both Avengers (early storyboards confirm Widow replaced Wasp) and Age of Ultron. The tiny heroes being founding members of the team, along with Pym creating Ultron it all seemed fishy to exclude them from both films. I have a feeling the reason was Edgar wanted to tell the origins within his film, which kept getting delayed due to Wright's other projects and constant script revisions. Now, with Wright out of the picture it looks like at least one of the Ant-Man stories connected to the Avengers can be explored in Captain America: Civil War.
In the comics Hank Pym had a huge role within the Civil War storyline, training meta-human recruits at Camp Hammond. Something I believe was already teased with set photos from Age of Ultron. I wouldn't be overly shocked to see Scott Lang played by Paul Rudd, actually take over this persona along with Pym's daughter Hope played by Evangeline Lilly. Pym used both the personas of Yellow Jacket (will be seen in Ant-Man) and Giant-Man in the Ultimates, so it's possible these could show-up in the films too. We might finally have some heroic characters joining Team Stark.
It should be considered that we've come to expect crossovers with these films, and this Captain America sequel is our last chance for an Avengers-type story until the cosmic epic Infinity War. It's also odd that while we'll be seeing a Guardians sequel, Marvel didn't even mention an Ant-Man sequel as a possibility. Since the cast signed multiple-film contracts it's expected they will appear in a bunch of different movies, without a sequel announced we have to assume they'll show-up in other Marvel movies.
You might be saying to yourselves, hey we've already have plenty of people returning for this Captain America sequel. To tell you the truth, Civil War's story lends itself to a massive cast and we could end-up seeing something even bigger than we've seen previously with Avengers installments. It's also possible Black Panther might be the only new character getting his origin told in the film, leaving enough room for established characters to mingle.
That's unless Marvel does indeed feel the need to include Carol Danvers aka Captain Marvel into the lineup. We've heard that Danvers could be a member of Team Cap at the end of Age of Ultron. Again, this film can handle a huge sprawling cast.
The only two that make sense at the moment are Maria Hill who is revealed to be working for Stark Industries at the end of Winter Soldier. Along with Stark's super-science buddy Bruce Banner, who will likely take the side of his best friend. If rumors are to believed via BadassDigest Cap's side will include pretty much everyone else from Age of Ultron. Such as Hawkeye, War Machine, The Vision, Quicksilver, Scarlet Witch and even possibly Captain Marvel.
Not to mention possibly lining-up a starring role in Ghost In The Shell, which will likely keep her out of the production. Her death in Age of Ultron, might also explain why talk of a Black Widow film has been essentially erased lately from the spin-off table. Even when Feige talked about the Widow film recently, he referred to it as a prequel, a clue to Natasha's fate?
I'm sort of thinking Hulk or Ultron ends-up killing Black Widow in Age of Ultron, this shocking event might be enough to spark Civil War. Banner lost control on the helicarrier in Avengers and almost killed Widow once before. Let's not forget Natasha was killed by a team member in the Ultimates comics, and those comics have been influencing these films a lot. She's been revealed to the world and her death could see the government react with the Superhero Registration Act, headed-up by Tony Stark.
If by the hands of Hulk it could also lead to his Planet Hulk banishment that's been rumored for ages now. Pym (or Scott/Hope) might take the place of Dr. Reed Richards in the Illuminati lineup. Since, we'll likely have Doctor Strange, Black Panther and Black Bolt established around Infinity War's release.
In the case of Thor, Age of Ultron reshoots are said to include Idris Elba's Heimdall and Tom Hiddleston's Loki, will likely see Thor too busy with Asgardian affairs to worry about the conflict back on Midgard. I assume these scenes are being shot to setup the events of Thor: Ragnarok, which could be happening during Civil War. The assumption is that Guardians of The Galaxy 2 and Ragnarok, will be planting seeds if not entire plot points for Infinity War Part 1.