Chris Evans as Steve Rogers/Captain America
Robert Downey Jr. as Tony Stark/Iron man
Chris Hemsworth as Thor
Mark Ruffalo as Dr.Bruce Banner
Jeremy Renner as Clint Barton/Hawkeye
Scarlet Johansson as Natasha Romanova/Black Widow
Patrick Wilson as Hank Pym/Ant Man
Stana Katic as Janet Pym/Wasp
Samuel.L.Jackson as Nick Fury
Clark Greg as Agent Phil Coulsson
Lou Ferrgino as Hulk
Keith David as Super Skrull
Sebastian Stan as Bucky Barnes/Winter Soldier
Derek Mears as Skrulls(voices)
Cameo:
Idris Elba as T"Challa/Black Panther
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Joss Whoeden or Zack Snyder as Director
Some ideas:
When Thor(maybe)defeted Loki in first movie some portal open on earth.In the beginning of the movie Skrulls come to Earth.For help Nick Fury must have two more heroes in the team.So Wasp and Ant man confirmed that they want in the team.One mistery was when Captain America see Bucky alive in 21st century.But Bucky is villian in the movie.The team defeted Skrulls .Then defeted Bucky.He told to Cap that Red Skull bring him life and made him a Super Soldier.Now Bucky is real dead.At the end of movie we see how Black Panther been called in the team.The End.
Editors Note - Use pictures :)