it's mostly a fan theory but they've been teasing skyes background for awhile now.
With "A Hen in the Wolf House," "Marvel's Agents of S.H.I.E.L.D." dropped some major clues for the direction of this season. In a dramatic confrontation between Skye and Coulson, he suggested that she may not be entirely human -- that she may, in fact, be an alien. And, well, he certainly isn't wrong. Expounding on my previous theory, here's a few more clues that point to "Inhumans" taking center stage in the Marvel Universe
But will the casual movie goer be? I mean, Mark Ruffalo doesn't sell tickets, throw in a Hulk story line that only comic fans know of, and is that going to make a ton of money? I don't know. In ways, the same can kind of be said for Guardians...but it had bigger names than Ruffalo.
I think enough casual fans know about the hulk for him to carry a solo movie.
Even if he doesn't the Marvel name will bring in enough box office. The only thing I really see stopping marvel is if people get superhero fatigue with all these studios now wanting a huge shared universe for their comic films including a lot of spinoff films.