Collider has the synopsis for Captain America: Civil War, which starts shooting April 1st in Atlanta. This synopsis is what went out with a casting notice, so it's probably not quite what we're going to see in the official press materials - ie, I think this is very vague and probably not written with a real knowledge of the script.
“Following the events of Age of Ultron, the collective governments of the world pass an act designed to regulate all superhuman activity. This polarizes opinion amongst the Avengers, causing two factions to side with Iron Man or Captain America, which causes an epic battle between former allies.”
Generally it's the story of Civil War from the comics, although the global aspect is interesting. This could be how Black Panther gets involved.
Now let's get down to business. Yesterday Bleeding Cool ran a scoop about Civil War, and who will be in it. Either the scoop is very wrong or it points to Marvel changing up some aspects of their post-Ultron plan leading up to Avengers: Infinity War. Here's what the site had to say:
So, Captain America: Civil War. It will feature Captain America, Iron Man, Black Widow, Hawkeye and Black Panther.
It will also feature Hulk, Thor, War Machine, Scarlet Witch, Wasp, and a fourteen year old Peter Parker, Spider-Man.
That first list is true and confirmed. Those characters are in the film. The rest of it? Well, here's where we get into soft spoiler territory. Go no further if you fear spoilers.
I am familiar with a couple of drafts of the script (one pre-ScarJo pregnancy, one post) and I can tell you that in both drafts the movie ends with the original Avengers splitting up and going their separate ways. Captain America and Nick Fury put together a new team, missing most of the big solo guns. Iron Man is done, Hulk is missing and Thor has returned to Asgard to deal with the events leading up to Thor: Ragnarok. Those last two are vital - my sources tell me they're sitting out Infinity War Part I as well, which means Marvel wants to take them off the shared universe board until they (along with Captain America) return in Part II. Bringing Thor right back for Civil War makes no sense. Bringing Hulk back makes even less sense, as his disappearance is a pretty big deal at the end of Ultron, and undoing that immediately seems silly.
Could these characters cameo? Sure, anything is possible. War Machine and Scarlet Witch are Avengers now, so their appearance in Civil War makes strong sense. Peter Parker? Maybe, but I wouldn't hold my breath on him having a major role. Ant-Man and The Wasp? Could be! There will be a Wasp in the Marvel Cinematic Universe. But here's the thing about this rumor: it's just listing all the characters who have been in the movies. On some level I get that Civil War would bring in a lot of cameos (it was being called Avengers 2.5 in-house), but since the plan for Infinity War is so specific - a lot of the name brand Avengers will not be in Part I - I don't see Marvel just shoehorning everybody in here.
And the most telling thing about this rumor: where's Winter Soldier? I have been assured again and again by sources deep in Marvel that Winter Soldier continues to play a major role in Captain America: Civil War. Anyone with legitimate inside knowledge of the film would have mentioned him.
Finally, Bleeding Cool reports that one Avenger won't make it out of Age of Ultron. You can figure it out by simply looking at the list of characters above. Here's where it gets weird: in one draft of the script Quicksilver did die. But in the latest draft of which I know he does not. I'm not sure what caused this change of heart - the plans for Infinity War? Whedon taking a shine to the character? - but as far as I know he does not kick the bucket in the movie.
These things do change, and if he was slated to die once it's possible he could still be killed. I'm also noticing that this rumor doesn't have The Vision appearing, despite him being on the team at the end of the shooting draft of the script (but Ms. Marvel was also there, so some of that has probably changed in the eighteen months since I had access to this).
Generally I simply do not believe the rumor, despite its vagueness meaning that some of it will certainly be true. This is the main problem with rumors when it comes to these movies - the general outline of the universe is clear from upcoming film titles and comic storylines, so it's easy to make up some convincing stuff that gets clicks.
What is my guess as to the shape of the MCU, Phase Three? I think that after the events of Civil War Captain America is no more - either dead or retired, possibly taking on the Nomad name. That film is very much setting up the next Avengers in a big, big way. After Ragnarok Thor is also out of the picture. The Hulk will remain missing or will pop up offworld. Cap's side will have lost the CIvil War, so maybe Black Widow and Nick Fury have gone deep, deep underground. Considering his general nature and the storyline of his solo movie, Ant-Man would also be on Cap's side if he's in Civil War, so he and the Wasp could join the spies on the lam. Iron Man is running the show when Infinity War Part I happens... and everything goes bad. It isn't until Part II that all the big guns come back, teaming up with the new kids like Doctor Strange, Spider-Man, Captain Marvel and a whole slew of Inhumans, to take down Thanos and save the universe.
Warning: a lot of that is based off a mixture of hearsay, insider information and informed speculation. This isn't a news scoop. But I think that's the larger story of the Marvel Cinematic Universe for the next three or four years.