Back in 2013, Incredible Hulk director Louis Leterrier revealed that Mark Ruffalo was his first pick to play Bruce Banner, but Marvel and Lou Ferrigno insisted on Edward Norton because they felt that he was right for the role. Most people think of him as a less confident and handsome actor. He was planning to reprise his role in the Avengers, but he rewrote too many rewrites and was considered a control freak by Marvel Studios. So they fired him and replaced him with Mark Ruffalo. That is why fans prefer him because he is more meek and confident as Bruce Banner. Terrence Howard was originally planned to reprise his role as James Rhodes/War Machine in Iron Man 2, but he refused to take a pay cut and Marvel had to replace him with Don Cheadle in the sequel. In Iron Man 1, Marvel paid Robert Downey Jr. half a million dollars, fearing his substance abuse history would get out of hand with the other cast members while Howard was paid 9 times more of that. However, I believe if Mark Ruffalo and Don Cheadle were cast as the characters from the beginning, we would have not had Norton rewriting scenes every day, having too many rewrites for the first "Avengers" and the Incredible Hulk would have been quite better.
Terrence Howard refused to accept that he had to be paid less than he was in the first film now that the second film had Rhodey's part shunk and they did not have to pay the actor as backup. But perhaps if Don Cheadle was James Rhodes in Iron Man 1 from the beginning and Howard was cast as Moses Magnum in a small role instead, RDJ would have been paided more than Cheadle and Howard himself.
But the film would have still been dark/serious as the MCU's tone began to change after Disney purchased Marvel in 2009. I believe if Edward Norton and Terrence Howard were cast in different roles in the films, they would have been restricted to small roles in the films. Norton could pull off an pretty interesting Igor Drenkov, had we got to see the origin of the Hulk in the Incredible Hulk, Norton could certainly put on some Russian accent and all, but he would have probably ended up in the annals of discarded MCU villains, or characters beyond that one film. I believe Norton did a pretty decent job as Banner though, but i prefer a more meek and more confident portrayal of Bruce Banner.
It would have been very interesting, seeing Terrence Howard as Moses Magnum in Iron Man 1. Of course, Obadiah Stane still would be the main villian and Moses Magnum would have played only a small role in the film. Terrence Howard has actually done a few villain roles as in Dead Man Down, etc. So I believe if Moses Magnum was implemented in Iron Man 1 as a villain, Terrence Howard would be very interesting as the character while Don Cheadle did War Machine.
If the MCU introduced these characters, here is how they would have roles in Iron Man 1 and the Incredible Hulk.
Moses Magnum(who was a terrorist and arms dealer in the comics) would have portrayed him as a weapons liaison between Obadiah Stane and Raza in Iron Man 1, illegally aiding Stane in helping the Ten Rings get their hands on Stark manufactured weapons.
Igor Drenkov(who was a KBG agent sent to learn the secrets of the Gamma Bomb from Bruce Banner) could have been readapted for the MCU as Edward Norton's character as a scientist who helps with studying the Gamma radiation that transformed Bruce Banner into the Hulk.
To me, it would show that Norton and Howard would do much better jobs as these characters had Marvel Studios realised that the former was a problem actor and the latter refused to take a pay cut and expected a pay rise for $8 million for Iron Man 2. This would have given Ruffalo and Cheadle an opportunity to play Bruce Banner and War Machine in all 7 film appearances. Would these films have been rated much higher? Let me know what you think in the comments!