3. Just... Lots Of Punching
As previously touched upon, another Hulk movie (in which the Hulk side of Bruce Banner's personality is prominently on screen) could only really end one way - Hulk wins by punching things. There's no other way that Hulk (and, in particular, the Marvel cinematic universe version of the character) can come away with a victory against his antagonist - and that makes the ending to any potential Hulk movie very predictable. Like his very limited dialogue, this could become very boring, very quickly. This is why the Hulk really needs supporting characters from hereon in. He's done the beat-down thing - he did it to Abomination, Loki and the Chitauri and it worked in all cases. Now, the Hulk either needs to be the last resort and/or solution after other methods have failed, or he needs to try his brutish methods, fail in doing so, and have someone else defeat the enemy with their wit, magic or speed (or whatever else his team mates have going for them).