Re: "Mr. Stark, you've become part of a bigger universe."
I like the idea of Cap already thawed out and recuperating/training to ge back to 100% after decades on ice (outside of filming). Pym and Wasp would also already be members, being internal to SHIELD (also outside of filming). As a backstory... as Cap nears readiness to return to the field, info about some real nasty bad guys with crazy resources, tech and power starts to surface (Madarin, Zemo, Red Skull, even Killmonger, etc). The idea for a super team emerges and Fury starts looking around to add more beyond cap, Pym and Wasp. He has his eye on Banner, and finds out that Stark is flying around. He sends that guy from the IM movie to check it out. Since Stark reveals his ID, Fury just makes his move.
Then they approach Ross to track down banner again. So this is the "larger universe".
I don't know if they can mention characters whose rights are owned elsewhere... But somthing like, saying "there are masked vigilantes, teams of genetically enhanced beings, and a slew of others with super powers out there, all with their own agendas. Some good, some bad. Uncles Sam decided it was time to prepare a team capable of responding to any threat by super beings."