I was brainstorming with a friend, and since a standalone Hulk film would be difficult to pull off, we'd like to see a Hulk Vs Wolverine, or Hulk Vs The Thing, film. both of which had been done in comics. Obviously, unless those rights revert to Marvel, we'll never see either of those crossovers.
Not sure how you could stretch that into a two hour movie though. There's only so many times you can watch the same two people fight, and even Hulk would eventually clue in that he was fighting one of the good guys.
Unless you have him under some evil influence the entire time or something.