There are lots of characters to choose from. Here are some that are from the movie:

• Captain America
• Iron Man
• Black Widow
• Hulk
• War Machine
• Rocket Raccoon
• Nebula
• Okoye
• M’Baku
• Ant-Man
• Hawkeye
• Valkyrie
• Wong

Don’t feel the need to only choose from this list, though. I’m open to alternatives (like Jane Foster becoming Thor while the original Thor is “unworthy”, or Wasp working alone while Ant-Man is under house arrest). The Avengers also has a long, long list of members in comic history.

These characters are explicitly unavailable:

• Captain Marval (in space)
• Loki (dead)
• Heimdall (dead)
• Gamora (dead)
• Vision (dead)
• Black Panther (disintegrated)
• Spider-Man (disintegrated)
• Doctor Strange (disintegrated)
• Winter Soldier (disintegrated)
• Falcon (disintegrated)
• Scaret Witch (disintegrated)
• Star-Lord (disintegrated)
• Groot (disintegrated)
• Drax (disintegrated)
• Mantis (disintegrated)
• Nick Fury (disintegrated)
• Maria Hill (disintegrated)
• Shuri (disintegrated)