One of the most fascinating aspects to MCU for me is the character development. Each character is highly unique, has a unique back-story that makes them who they are today. Sometimes they share similarities and sometimes their traits clash. This is nothing new in the world of entertainment, but for some reason Marvel is able to do this in a way that shows the contrasts in bigger fidelity.
Maybe these have already been mentioned. I know time-travel has.
The top few floors of the Baxter building are a decades-old "Fantastic Four Museum", closed for quite a while, originally made in homage to the first family, which has been missing and forgotten since the mid sixties.
A time rift opens, transporting the Fantastic Four into the museum together with a spherical chunk of their lab and half the Fantasticar.
Upon the whole world finding out that the first family has re-appeared, their love for the golden age begins to spark anew. The Four are visited by Stark, who offers to fix the lab and car, and asks where they've been, to which Reed replies that they've been traveling through alternate universes and timelines. There's an obvious infatuation that Tony has with Reed, having grown up hearing stories about him from his father. Stark Tower's logo was modeled after the first family's "4" logo, which eventually inspired the Avenger's A. Stark is a bit taken aback by the thought of alternate universes, but trusting Reed that its true, asks what its like, to which Reed replies "Have you ever met the most ridiculous version of your self you could ever imagine?" (a tongue-in-cheek reference to all the rebooted films). Stark shrugs, "Two of me doesn't sound that bad."
The Fantastic Four look and talk like they are from the 60's and Beatlemania-like hysteria begins to grow around them. People are enamored with real-life heros from the golden age and expectations are high that the Four will solve all the world's problems. Conspiracy theorists are sure that the world's problems have come about because of the lack of technilogical advances Reed and crew would have brought to the table.
Stark finds out a different story, though, when helping to reconstruct the lab. The first family are constantly at each others' throats, making the Avengers' arguments looks like polite disagreements. They throw things. They name-call. They resort to pettiness. All the while, performing their tasks to get the lab and Fantasticar up and running.
When a second time rift rocks Manhattan, Stark sees them spring into action to quell the destruction and calm the people, working together like a well-oiled machine, despite their on-going cat-calling and put-downs, which they sucessfully hide from the public eye.
This intrigues him, especially in light of the events of Civil War.
The second rift brings Dr Doom, who has been following the 4 through alternate universes and timelines.
As Stark comes to grips with the idea of alternate realities, Dr. Doom prepares to mold the earth of this timeline into his own image (with some kind of evil plan).
The Four work like clockwork to make a plan to defeat him, all the while conintuing with their insults and nitpicking. And Stark confesses that he may have made the wrong choice about the way he treated Captain America. Reed and company are shocked to hear that Captain America is alive. Apparently, he is as much an idol to them as the rest of the world has projected onto the Fantastic Four. Stark, knowing that the Cap is not the smartest one of the Avengers, struggles with a bit of jealousy.
In the final battle, the 4 fight to the near death to defeat Doom, a fate they knew was inevitable. As they are whisked away to be treated, Tony has a deeper understanding of loyalty, that it transcends petty differences, and even life itself.