I like the idea of a movie about the "new" Avengers. Rebuilding/reorganizing the team after the loss of a bunch of key members. In the comics, every super team goes through significant cast changes fairly regularly as it allows the writers to bring in new storylines, new team dynamics, etc. As I see it, here are the options they have, although they can definitely add other characters.
War Machine- I like Don Cheadle a lot, and think the character has a good combination of leadership and dry sense of humor. He will clearly be involved.
Captain (Falcon) America- They clearly set him up to be part of the new franchise. I think the character is kind of boring, but the combination of wings and shield will be pretty cool.
Dr. Strange- Maybe the most interesting remaining character (both in personality and powers). Seems like a no-brainer. Hopefully that is the case.
Scarlet Witch- I think I like her, but they have really been inconsistent with how powerful she is and what she can/can't do. She's attractive, so that is a plus. They also haven't given her a lot of screen time, so I'm not sure she has the acting chops to be more than a background personality.
Ant Man- Great character with cool powers. Clearly needs to be involved
Wasp- I hope she is part of the team, primarily because AM and the Wasp were both members of the original Avengers. Plus, Evangeline Lily is hot.
Spider Man- Seems likely that he will be involved. Fan favorite. Cool to watch in action.
Black Panther- Seems likely to be involved, if only to keep the black fan locked in to the franchise.
Captain Marvel- They have set her up to be a key member, but maybe it won't happen. I think Brie Larson is boring, and they clearly have issues with CM being too powerful and making every crisis too easy to fix.
Hulk- Still a possibility. Maybe too injured to fight effectively and takes over as the home base scientist role on the team.
Winter Soldier- He's kind of meh, IMO. But, what else is he going to do with himself.
There are other, secondary, characters that could be included, but I have already listed too many, so I doubt any of these play a significant role (Shuri, Pepper Potts, Valkyrie, Hawkeye, Wong).
It will take some work to come up with a good mix of powers and personality to form the team they want. It will also take some work to come up with explanations as to why the characters they cull are not involved. For example, maybe Scarlet Witch is traveling through time to be reunited with The Vision, so she is not available. IMO, it doesn't work to say, "Wanda decided she just doesn't want to do this anymore, so she is getting her degree at Villanova".