Universal still own solo Hulk rights 

Fans have been screaming for a solo Hulk movie since Mark Ruffalo first smashed his way on to screen.

Whilst doing press for Avengers:Age of ultron, Ruffalo revealed why we haven’t yet seen a Hulk movie. 

“As far as a Hulk movie, a standalone Hulk movie, Marvel doesn’t really have the rights to that yet. That’s still Universal’s property, so there’s that issue. That’s a big impediment to moving forward with that. Now I don’t think that’s insurmountable, by the way, but I don’t know where it’s going from here for me”.

Because Universal has distribution rights for a solo Hulk film, that does make a solo film difficult for Marvel. But a few years ago, we thought a Sony/Marvel deal was a pipe dream so a universal/Marvel deal isn’t impossible.