Agent Coulson
Agent Phil Coulson was the "glue" of the first phase of the Marvel Cinematic Universe, so what better way to usher in Spider-Man's inclusion than to have Coulson show up in the next Spidey film?
If Coulson geeked out over Captain America, there's no telling how giddy he'd be about the friendly neighborhood wall crawler. Plus, the dynamic between the two characters would be incredibly fun to watch.