Joss Whedon takes us beyond the prelude into the expansive, going international and intimate. If the first film is the team's blind date, the second film is the real relationship. They've gone from being in the same space to meshing into the collective team that we love. This integral sequel brings their mandate, the headquarters, the interplay, the familiarity and frictions, the gestalt group.
The Big 3—Iron Man, Thor, and Captain America—gain new dimensions in this ensemble, but the characters without solo films especially graduate into full being. Hawkeye provides the unexpected haven that grounds them. Hulk and Black Widow bond over the lack of control over their core identity. The damaged Quicksilver and the Scarlet Witch crave vengeance. And The Vision arrives fully-formed yet soul-searching.
Studio franchising is the prime interference and achilles heel of graphix movies. Whedon warps impossible contortions trying to accomodate studio-mandated set-ups for Thor 3, Black Panther, and Captain America 3 amid his own main story. But he pulls it off, barely, through compression, editing, and sheer force of will. (And there's always the deleted scenes on disc for grumblers.)
The definitive line-ups of the Justice League and The Avengers in people's minds are the classic 70's teams. Essentially, no one can think of The Avengers without including Iron Man, Captain America, Thor, The Hulk, Hawkeye, Quicksilver, Scarlet Witch, and The Vision, along with Black Widow, Ant-Man, and The Wasp. Whedon completes the classic team here with Vision and the Witch, while cycling future turnover with Falcon and War Machine (with Ant-Man and The Wasp pending).
You can tell a Marvel film is doing right when it evokes its founders. The lifting of Sovokia by vast tech, with villagers running and heroes fighting robots, recalls the classic Stan Lee/Jack Kirby arc of FantasticFour #84-88 (1969), where Dr. Doom lifts his country Latveria on Kirby-tech. One of the joys of comics is that they can create spectacular things that films could never do. And one of the joys of this era is seeing film finally honor those things right. The trick is to make it believable emotionally and visually, and Whedon has syllabused the master classes for it.