Marvel is still kicking so much ass right now. But a thought occurred to me. They have this massive overarching story with Thanos building up, but what about after that? In Guardians of the Galaxy they introduced Knowhere and the concept of massive celestial beings destroying worlds. They could easily build toward a massive Galactus story line.
Problem: Marvel doesn't currently own the movie rights to the Fantastic Four and several of the associated villians like Galactus.
A lot of fans are trying to pressure Fox into selling the rights back to Marvel or getting into some sort of Sony/Spider-Man contract with them. But, if I were Marvel, I would just wait it out. They've had several of their characters revert back to their ownership without paying a dime. Punisher, Daredevil, Ghost Rider... Fox is failing miserably with Fantastic Four. Marvel has already discontinued the Fantastic Four comics due to lack of sales. There's not real incentive at this point to pay anything for the franchise. Marvel can essentially just wait Fox out.
Fox, on the other hand, faces paying $200 million for another bad movie in a 3rd attempt to create a successful franchise. They can try to sell the franchise back to Marvel in attempt to recoup some of their lost money. Or, they can do nothing, and the franchise license expires.
There's no rush for Marvel. They've demonstrated with Ant-Man and Guardians of the Galaxy that they can take less popular comic characters and build a successful movie with them. Actually, two of my favorite movies so far. There isn't much of a question that they could do the same with F4. And fans have had enough exposure now to characters like Black Widow, Hawkeye, War Machine and Falcon that it's not unreasonable to see them get their own movie. On top of that, with Doctor Strange, Black Panther and now Spider-Man back in the mix, and it's looking like Downey Jr is going to do an Iron Man 4, likely focused on the real Mandarin... there's no real shortage of the 'top shelf' Marvel brand characters coming to an end anytime soon. Marvel has time to plan.
If and when the F4 franchise and all the villians come back to their ownership, they can do a proper reboot. While, in the mean time, be building with small nods and easter eggs toward a Galactus story. There's no time table, and the more head nods they have at it, the more epic the story becomes over time. Fans will be able to reach back to Guardians of the Galaxy. It'd be pretty much a shoe in for a big Avengers flick. You have a F4 movie, Silver Surfer arrives as the herald of Galactus, F4 finds a way to turn him and then the Avengers get a movie where they are brought in to deal with the coming threat of Galactus. Done deal.
Just a really great idea of the top of my head.