I agree. I'm 100% confident that a Mark Ruffalo Hulk movie would work, but they would need to make a couple of changes from the previous two solo movies:If they did the Hulk properly he could definitely carry a movie, we're talking the proper actor, story line, etc...
1. Establish the more intelligent Hulk from the comics. Originally, the Hulk was pretty smart. He was able to read, had full command of personal pronouns, could hold down a job, stop an alien invasion, and was even able to maintain a secret identity while still in Hulk form. This was mainly back when Stan Lee was writing the character, but he also exhibited a high level of intelligence during Planet Hulk.
2. Stop acting like the audience is going to be surprised that Bruce Banner turns into the Hulk. We all know it's going to happen, so we don't need to spend half the movie building up to it.
3. Keep him in Hulk form for the majority of the movie. Somewhere between 90-99% of the film should be with the Hulk and Bruce Banner should only make one or two brief appearances. People pay money to see the Hulk, not an hour and a half of Bruce Banner whining about the Hulk. Planet Hulk would be a perfect storyline for this as well.
This is what I think would work (Hulk is my second favorite comic book character, so I have given this a lot of thought). The key to it all is establishing the more intelligent Hulk, though, without that any Hulk movie will just end up retreading familiar territory.
Maybe they can use the next two Avengers films to establish the intelligent Hulk and then shoot him off into space to lead directly into Planet Hulk, followed by World War Hulk.