Doctor Strange
The Pitch: "Harry Potter meets Nip/Tuck."
The Explanation: What happens when one of the world's greatest surgeons loses the full use of his hands in a car accident? If your answer is "He goes to Tibet and becomes the world's greatest magician," then you clearly know your Strange. We're saying, keep him as the arrogant bastard he was as a surgeon, and then let him get the shit scared out've him by some Guillermo del Toro-esque monsters, and audiences will come running. Marvel seems to agree; Kevin Feige has spoken often about Doc being a character he'd love to see being made into a movie.
Must Read: Brian K. Vaughan's Doctor Strange: The Oath is a great choice to get into the character.

Black Knight
The Pitch: "What if Martin Lawrence's Black Knight movie wasn't played for laughs and didn't suck?"
The Explanation: Simplify this Avenger's backstory considerably, and you've got the plot for a movie: The ancestor of a famous soldier during the time of King Arthur ends up, through magical process, back in that era and creating the legend that his ancestor was supposed to have personified. Yes, it's Hiro's plot from the second season of Heroes, but Black Knight did it first. And, let's face it, better.
Must Read: Essential Defenders volume 1 gives you some of the character's time traveling history.

The Pitch: "War Of The Worlds by way of Planet of the Apes."
The Explanation: Set in an alternate world so far out that it may as well be Middle-Earth, Killraven is the story of War of The Worlds Round 2: The Martians from HG Wells' original story have come back and enslaved humanity, forcing breeding so that they can eat babies (Subtle, this isn't) and otherwise just using and abusing humanity as they see fit. Only one man - Jonathan Raven, apparently called "Kill" to his friends - can save the human race in what can only be described as Battleground Earth done right.
Must Read: Essential Killraven volume 1.