I think I wrote about this before, but if I were writing at least the first Infinity War movie? It'd be a race. You mix and match various heroes and send them off in smaller teams after each of the Infinity Stones. Basically, it a race to collect them before Thanos does. And you do some interesting team-ups, like maybe Groot and Hulk, Rocket and Iron Man, Star-Lord and Cap, etc. Assuming they're including The Defenders, maybe Daredevil and Hawkeye (one's blind, one has hearing issues).I think (or even hope) that Cap and Bucky don't have a role in the Black Panther movie, or are just very minor supporting roles at best. Otherwise they'd steal the spotlight from T'Challa. Ulysses Klaw's already been introduced in the MCU, so I wouldn't be surprised if he's the antagonist. My guess is that the basic premise will be that Klaw's going after more of the Vibranium, and T'Challa will have to stop him.
And surely everyone's going to come back for the fight against Thanos (barring any deaths along the way). If Cap does shout "Avengers assemble" somewhere along the line, I'd hope it's in the final conflict, when everything seems lost, and he utters an inspiring rally cry that drives his allies forward for one last push. Man, I can imagine that scene.
Also, it wouldn't surprise me if Infinity War 1 and 2 have mostly different sets of protagonists, due to the superhero bloat in the MCU right now. So, for example, part 1 involves the more "grounded" heroes, like Cap and Iron Man and the rest of the human Avengers. Part 2 is when things get kicked up a notch, which would then require a more cosmic force to fight against Thanos. This is where the Guardians of the Galaxy, Dr. Strange, and the Asgardians get involved.
And at the end, they fail. Thanos slips the last stone into the Infinity Gauntlet, raises it victoriously...and everything fades to white as he rewrites reality. Credits.
The second movie is the original Infinity Gauntlet story (which started with Thanos already having the Gauntlet), done in an MCU manner.