Wanda:
Jean Gray
Scott Summers
Bucky:
GANDALF
Daenerys (I know Daenerys is the wrong book, but...I felt like it works)
Thor:
Peter Quill (As your idea of a joke)
Sylvie Laufeydottir (Girl Loki, he had no idea, but you had a dream...)
Loki:
Fairy Princess
Emperor
Bruce:
Captain Jean Luc Picard (Star Trek: Next Generation)
Lt Nyota Uhura (Star Trek TOS)
Steve:
Peggy Carter (Surprise dad!)
Red Skull (Almost gave him a heart attack, but pretending to be evil is FUN)
Tony:
Spiderman (Your funny idea of a joke)
Natasha:
Leia
Han Solo
Clint:
Katniss Everdeen
Robin Hood