Better people than me can speak on this, but I always thought the less bankable cats--Starfox Quicksilver, The Black Knight, Vision and Wonder-Man (especially those last two)--gave texture to the story. It's actually hard for me to imagine the Avengers without any trace of Vision's relationship with the Scarlet Witch.
I'm not so worried, though. I'm sure they'll introduce all of this in films subsequent sequels. It's a know fact that adding more characters always improves comic book sequels.
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