Re: Who should be the villain in an Avengers sequel? (Poll)
The Ultron story is resonant because it takes a generally good man, who happens to have a rather large inferiority complex, and deconstructs him completely. Marvel can't (and shouldn't, and won't) do that to their lead character, and without that element Ultron is just another evil killer robot.
It's not that it would be hard to tell an Ultron story without Hank, it's that an Ultron story is empty and weightless without Hank. Stark makes a robot, the robot goes bad, Stark helps the Avengers beat the robot. We end back on square one, we've told a story everyone's seen 100 times, and no one cares.