Note: This is not an origin movie, but a continuation from Avengers 2, assuming nothing major happens to the main cast.
Mark Ruffalo as Dr. Bruce Banner/Hulk
Ruffalo has portrayed the most successful film adaption of everyone's favorite giant green rage monster. Have Lou Ferigno provide Hulk's voice like he's been doing. So this casting is without a doubt.
Jessica Stroup as Betty Ross
I've seen her cast for She-Hulk but I think she'd be better fit for Betty Ross in a film adaptation. Hulk's main love interest in a possible Hulk trilogy. She eventually becomes the wife of Banner and one of the few people that can return him to his human state after he transforms into The Hulk. No Red She-Hulk for Betty this time around.
Jeff Bridges as General Thaddeus E. "Thunderbolt" Ross/Red Hulk
Reasons: As far as I know Jeff has no issue changing his appearance for movies. Every other actors I considered was already Thunderbolt which I learned recently due to the other Hulk movies being God-awful. Jeff Bridges could be a good General Thunderbolt because he could step out from the ranger umbrella and play someone slightly different but with some tweaks.
Angie Harmon as Jennifer Walters/She-Hulk
As she already plays a lawyer on Law and Order it wouldn't be a stretch for harmon to play the cousin of Banner. A possible stand-in or CGI may even be neccessary for the She-Hulk transformation but overall I feel Harmon is the best choice. A human that requires a blood transfusion from her blood cousin, Bruce, his blood affects her with a similar condition that allows her to become a Hulk-like creature. Sher gains super-strength but however keeps her intellect and consciousness. She is able to reason with Bruce is in his transformed state and calm him down.
Ron Perlman as General John Ryker
A human obsessed with the capture of The Hulk and also many other creatures and superhumans rumored to exisr, Ryker is the sole reason the military keeps up with its' attempts to capture the giant green monster while Ryker also makes futile attempts at making out the creature's identity. Ironically, he sometimes works closely with Bruce Banner.
Sean Penn as Dr. Stephen Strange
In an imginary post-credits scene we see Sean Penn as the mystical Dr. Strange confirming the character for future films.