Last month we learned that Marvel has a slate of films planned out through 2028. That's a lot of freakin' movies, and so it wouldn't be too far-fetched to think that another solo Hulk film could happen.
While talking to a local radio station in Minnesota, the original Hulk himself, Lou Ferrigno, said that Marvel is planning on another one. Here's his exact quote:
"Now because he saved the day, they're making another solo Hulk movie after the second Avengers comes out."
Ferrigno provided the voice of Hulk in The Incredible Hulk and The Avengers. He also recently revealed that he will be back for Avengers: Age of Ultron. I'm sure he has some inside information on what's going on.
Another Hulk movie will be made, it's just a matter of time, but what Hulk story Marvel is going to tell? I'd personally put my money on Planet Hulk. It was already rumored last year, and even though Marvel and Joss Whedon denied it, I'm standing by it. Of course Marvel is going to deny it! But let's just wait and see what happens with Avengers: Age of Ultron. How awesome would it be if it set up a Planet Hulk film?!
Whatever Marvel is looking to do with Hulk, one day it has to lead to Planet Hulk. That story needs to get the big screen treatment!