I would make the sequel darker. I want the main theme of the film to be "control", both Banner and Hulk fighting for it. Banner realises that his problem has become worse because minor things that make Banner angry make him transform. Hulk is in Banner's head, waiting to grab on the little things that make Banner tick so he can get rid of Banner for a period.
Have more "Hulk hates Banner" and "Banner hates Hulk".
Show more of Banner's multiple personality disorder. This can be perfectly shown with a scene of Banner going to Samson for help and Samson kind of puts Banner into some kind of hypnosis so he can explore Banner's mind.
Have The Leader as a villain in the shadows. Make him mysterious. Ross finds out that Blonsky was actually a Russian spy who tried to get American Military secrets and the whole British Special Ops thing was just a con. So Ross is livid! He gets Stark to help out with a project Stark has been working on called "HB-2000" (Hulkbuster). This could follow on from The Avengers film when Iron Man and Hulk battle and Iron Man is getting killed. Cap asks something like "Haven't you got a bigger suit?" and Iron Man replies "Yeah, in my workshop!".
The Leader breaks the Abomination free and manipulates him into thinking Banner made Blonsky into a monster so he cannot change back. Abomination goes to Russia to find his wife who he accidentally kills and realises he is a monster so wants to kill Banner because Banner can change back and be with Betty.
Big brawl between Hulk and Stark in Hulkbuster suit
Big brawl between Hulk and Abomination.
Hulk confronts the Leader.
And in the third film it is just The Leader vs Hulk.
Can you imagine them following the sequel from where the first film ended? Like it had Banner meditating then he opens his eyes showing they are green and has a smile on his face. Carry this scene on longer but that smile turning into a devilish grin and then exploding into anger with Banner roaring "Puny Banner!!!" in a deep Hulk voice whilst he transforms.
This just shows how desperate the two are for control.
I have a lot of ideas in my head but can't really get them out onto words without this post being incredibly long!!
Okay Sarg, I like where you're mind's at. One question: Since you've got Hulk and Abomb fighting again, how would you make this fight different?
I like the multiple-personality angle (they really misfired by not developing that even a little bit in TIH). So flesh it out a sequel...
Much as I like Shulkie I don't think she's movie material.