Well in the Avengers their are 2 normal ( 3 if you include Iron Man) humans fighting against aliens alongside metahumans. Just because we never seen it or hinted about it. Doesn't mean metas don't exist. Actually, Tony Stark is similar to Batman. A wealthy man who uses his smarts and wealth...
Avengers has already done over 200million internationally and is projected to be over 500million with the US opening by Sunday. I know Warner has toyed with doing a Justice League movie. If Avengers does what its projected to do. Warner would be foolish not to looking to get a JL movie going...