In the tradition of Marvel's What If series, create your own ideas. Feel free to expand on your own and others as you figure out the ramifications of these What Ifs.
I'll start:
What if Doctor Doom was part of the Fantastic Four?
What if Hank Pym and Janet Van Dyne never met?
What if Tony Stark never got that shrapnel in his heart?
What if Captain America fought in World War One?
What if the Hulk never quit the Avengers?