4. Sentry
Although the story would have to be altered, given the sheer number of characters in the comics, Sentry attacked Asgard in the "Siege" arc and Thor had to kill him. Siege saw Norman Osborn (who is, of course, tied up in the Amazing Spider-Man franchise) lead his team of Dark Avengers in an attack on Asgard, declaring it as a threat to national security. Sentry leads the way and attacks first and, frankly, that alone would be enough of a threat in a Thor movie given how powerful he is and what his alter-ego is. His alter-ego is the Void - and during the Siege storyline it managed to take down Asgard by itself, defeating everyone who opposed it including killing an amped, Norn Stone-powered Loki with complete and utter ease. After that, it battled Thor to the extent that it reverted back to Sentry who, realising what he'd done and knowing he was unkillable unless he chose to die, asked Thor to kill him. Thor reluctantly obliged. It would make one hell of a movie if an organisation like HYDRA or A.I.M got Sentry on side and, wanting to eliminate Thor, sent him to attack Asgard - and the Void coming out would be an added bonus because it would look awesome in live action.