I'm just going to get right to it.
Captain America: Jensen Ackles
He's already proven he can be a bad ass as Dean Winchester on Supernatural, and he was actually originally considered for the role, and he certainly has the build to be a super soldier, so I think it would be kinda cool to see him as Cap. Who knows, maybe Chris Evans will end up joining Jared Padalecki.
Iron Man: Bradley Cooper
I think he'd be perfect as Iron Man because he's already proven he can be funny and sarcastic in The Hangover. He can also be very serious when he needs to be as evidenced by his performance in American Sniper. Also, he's already been in a Marvel movie, as a tech whiz, I am speaking of course of his role as Rocket Raccoon in Guardians of the Galaxy, so I think he'd fit in a suit of armor quite well.
Thor- Stephen Amell
Yes, I know he's already in the DC universe but bear with me for a moment. He already looks good with long blonde hair as we've seen when he was on the island. He certainly has the build to be Thor, and he's proven to be a total bad ass who can take and deal a beating as the Green Arrow.
Hulk: Sean Maher
I think he'd make a pretty good Hulk considering he has already shown that he can be that sort of closed off, really brilliant scientist on Firefly. In the Avengers, Bruce Banner is portrayed as someone who really doesn't like where they are i.e. The Helicarrier, and all he wants is to be left alone. I still have suspicions that Joss Whedon just took Simon's character and stuck him in the Avengers. Also, an actor who was on Firefly, in a movie directed by the creator of Firefly, would just make the world a better place.
Black Widow: Yvonne Strahovski
Ok, after being Sarah Walker on Chuck I'm surprised she didn't get every good super spy role ever. She clearly has the looks to play agent Romanoff. Just to say it again because it isn't important enough, she has already been a super spy who has trouble expressing their emotions, I think she'd be PERFECT as Black Widow.
Hawkeye: Colton Haynes
Yes, I know I'm putting two Arrow actors here, but hear my case. Colton Haynes has already showed us he knows how to handle a bow as Roy Harper. Also, he knows how to give off that lone wolf vibe as he is very distant with Thea in season 2 of Arrow. He is also clearly built to play the archer and I think it would be nice for this actor to get some more credit as I feel he is under appreciated.
Nick Fury: Will Smith
I know that re-casting Samuel L. Jackson may seem impossible, and it is, but I think I've come pretty close. Will Smith while he is known for comedies, has also had very serious roles and has shown that he knows how to take charge i.e. After Earth. He has also proven himself to be a worthy action hero who knows how to fight i.e. Independence Day and Men in Black.