Are non of you people creative? LOL, ok so you've never seen a movie where the supporting cast is larger than life and has powers? Ummmm X-Men? And those films still managed to focus on Wolverine, Charles and Magneto. And even if X-Men didn't exist.
You guys had never seen an Avengers movie before 2012 right? You guys had never seen an Iron Man movie before 2008 right? You guys had never seen an Australian surfer like dude bro play a legendary villain before the Joker in the Dark Knight right?
People always get scared and worried and think it'll never work when somebody tries to do something different and unique. Everyone thought The Avengers and the MCU would never work, no studio would try it. Marvel did it and shut everyone's asses up. Everyone thought two giant studios could never work together and play nice but Marvel proving b*tches wrong again by bringing Spidey to the MCU. Everyone thought the novelty of seeing these heroes standing together would ware off and Avengers 2 would fail, Marvel proving MF's wrong AGAIN!
Marvel are not perfect by any means. But they are trying to innovate, making larger than life main characters into supporting cast members is a unique approach and I for one know how I would do it. You just freaking do it! Simple as that.
You don't worry about screen time, you don't worry about any if's and buts you just write a story from Captain America's perspective and everyone around him reacting to it. It doesn't matter if the supporting cast is "Punk kid 4" or God. You just write the story in a way that makes sense to cap and everyone around him but just focus on cap.
It's really not that hard guys.