I don't think that it was the character that had anything to do with the success or failure of the film "Green Lantern", but rather it was the marketing. The film was initially written and produced to be like Iron Man. Hal Jordan was to be a cocky, irresponsible person who fell upon a ring with great powers. His girlfriend was the head of a company that manufactured weapons for the defense industrial complex and the story surrounded Jordan's struggle to show courrage and accept his role as a hero (kind of like Iron Man). The true fans of the comic after seeing the first trailer were not pleased with the story line and wanted to see more of the Green Lantern Corps. In the follow on trailer the producers made it look like the Corps would be featured in the film, but in actuality, they were sparingly used. The marketing was confusing, the film was not quite as advertised and received poor word of mouth (or word of text). Had they used John Stewart instead of Hal Jordan, I seriously doubt that it would have changed anything. Thus, we shouldn't really looking at a character change as a solution to the real problem.