Time to speculate wildly!
If you want to come into the movie with a blank slate... you should probably skip this conversation. But for those of you that know the comics, or don't mind spoilers, let's chat about these trailer breakdowns! The highlights: Scarlet Witch can fly, Iron Man might be getting some cool new armor, everyone is upset. Of course, a few things will be different from the comics...
-Secret identities probably won't matter as much
-No X-Men or Fantastic Four
-Unlikely to get the Stamford Incident
Which is mostly due to licensing issues and time/budgetary constraints. Plus, while in the comics Civil War set up Bruce Banner's Planet Hulk arc, it seems like Age of Ultron did that job already
Yeah, I'm weirdly excited for this character's return. Blame Frank Grillo. Despite the huge ensemble cast, it does feel like this movie might genuinely be a Captain America movie. The thematic emphasis on freedom and de-regulation. Bucky's role seems to have been expanded from what it was in the comics. The return of Crossbones and Baron Zemo (two fantoms of Cap's past) and speaking of...
Who is Martin Freeman?
He was only there for a second, (with Sharon Carter) and Marvel is still keeping his identity a secret. But considering we haven't seen Baron Zemo yet, could Martin Freeman's character be connected to him somehow?
And yeah.... who do you think will switch sides?
We can probably eliminate Vision and Black Panther- they're too new to the universe to have an emotional impact if they switched sides and Vision's role isn't likely to be big enough. Rhodey or Sam would be upsetting for sure. Who do I think it will be? Bucky. Just hear me out: What would be more upsetting, more surprising- and yet more understandable? He suffered because Hydra was allowed to operate in secret, hiding within S.H.I.E.L.D. and some more oversight might have put an end to the covert assassinations he was forced to commit for them.
Plus that would put him on the same side as Black Widow, and let's face it, she's single again. I wouldn't put it past Marvel to try and make yet another romantic sub-plot work. At least this one would have some basis in the comics.