My thoughts. Some spoilers for Age of Ultron and some rumours:
Iron Man's Team:
> War Machine
> Scarlet Witch
> Vision
> Hank Pym
> Maria Hill
> Carol Danvers (Unpowered)
> Dr. Cho
Captain America's Team:
> Falcon
> The Winter Soldier
> Black Widow
> Ant Man
> Doctor Strange
> Hawkeye
> Sharon Carter
Black Panther will be undecided in the middle. Spiderman will be on neither side as well, and only appear as a small cameo.
Reasonings for the teams and how they'll all fit into the narrative:
War Machine is never going to leave Tony's side. Likewise Falcon will never leave Cap. These will be the primary sidekicks for each team.
Scarlet Witch has been rumoured to be siding with Iron Man, as has the Vision. Remember Sokovia had a strong anti-avengers sentiment, not liking them flying around doing whatever they please. So SW will get over her dislike of Stark and side with him. Vision is an android who does still like order. His relationship with SW will likely be explored.
The Ant-Men will pick opposite sides during this film. There are simultaneous rumours that Ant-Man will break Black Widow (on Cap's team) out of jail and rumours that he will side with Stark and become Giant Man. With both Pym and Lang both can be true. Lang can side with Cap and Pym will be an old science buddy of Starks. Pym may fill in Reed Richard's role from the comic, at least broadly.
Hill requires little explanation. Though honestly they could easily justify her going either way. Dr. Cho obviously won't be a superhero, nor in a major role. But she's another of Stark's science buddies and I liked her in AoU so I hope she returns. ETA: Also I feel that they wouldn't have had her survive AoU if they weren't going to use her for more films.
Carol Danvers has been rumoured to appear as a yet to be powered pilot. Probably not in a major role, but a few decent scenes on Stark's side to set up her own movie.
The Winter Soldier will be vital to the story. The big rumour is that he will be revealed to have been responsible for the death of Tony's parents. Sparking a lot of the conflict. Steve will try to reconnect with his old friend.
Black Widow goes without saying - she's Cap's buddy, she'll stick with him to the end. As I said above, the rumour is she'll be arrested only to be broken out of prison by Scott Lang.
I would have liked to see Hawkeye side with Stark to provide some juicy drama when he and Black Widow choose different sides. But the rumours indicate otherwise and it makes sense. My personal speculation is that he won't have a huge role, having retired to spend time with his family. But Cap's team will call in a few favours, possibly hiding out at the Barton Farmstead when they're forced to go underground.
Sharon Carter obviously needs to side with Cap to provide some romance. Hopefully doesn't become a romantic plot tumour and Sharon can make a bigger impression than she did (for me at least) in Winter Soldier.
And finally this is just my wild speculation but I think Doctor Strange may appear. We know a Dr. Strange villain - Baron Mordo - will appear. But what about the man himself? If you look at the two lineups above, Stark's team is vastly more powerful than Caps. Most of his team is just normal people with a few gadgets. So he calls in Dr. Strange to balance things out.