The reason is simple why the Hulk’s solo films have been less successful than the other Marvel heroes yet he stole the show in the Avengers is because with all due respect to him he’s really a One-trick pony.
Meaning that all the other heroes have to do is point him in the right direction, say ‘Smash’ and cut him loose.
But while that worked in ‘The Avengers’ it doesn’t let him hold a whole movie by himself.
Dr. Banner is indeed a complex character, but his alter ego is just a Tank.
I think to make a franchise of Hulk movies work is to have other colorful protagonists support the movie with him and interact with him as Bruce Banner while the Hulk is reserved as the Deus aux Machina for them to either fight with or against.
Such candidates would be: Rick Jones, Doc Samson, Jennifer ‘She-Hulk’ Walters, etc.
What do the rest of you think?
His movies would be a great way of introducing other Marvel heroes and giving them their moment in the spotlight.