A secret cabal of Earth's self-appointed protectors (consisting of Professor X, Black Bolt, Reed Richards, Iron Man, Doctor Strange and Namor) decide the Hulk is too dangerous to stay on the planet. So they trick him into a space shuttle and rocket him to the furthest-flung corners of the universe. He soon rises up against the oppressive, gladiatorial nature of the planet he lands on, and becomes ruler. Just as he's finding true happiness with wife and unborn child, the shuttle he arrived on self-destructs, killing all and sundry.
Somewhat understandably, he returns to Earth more than a little miffed, and systematically beats the living hell out of everyone who gets in the way of his quest for vengeance.
The gabillion dollar setpiece:
The Hulk's first attack on New York City, in which he defeats the New Avengers, the Mighty Avengers, the Fantastic Four, Iron Man, Ghost Rider, and destroys Stark Tower in the process.
Chance of happening:
Joss Whedon has already rubbished rumours
that the plot of
The Avengers 2
will lead into the initial rumblings of a potential Planet Hulk storyline, but that doesn't mean things won't have come together by the time
The Avengers 3