Ever since the comic book version of the Norse god was introduced in the Silver Age, Loki has been one of the most consistent villains for both Thor and the Marvel Universe in general. Known for his shapeshifting and devious ways, the Norse God of Mischief has taken on countless forms throughout the years with which to vex and outmaneuver his enemies.
The alternate reality of Earth X and its sequels saw Loki taking on countless new forms, from a darker version of himself to a featureless appearance and finally the visage of Thor himself. This later new look came complete with a new lease on life, resulting in a Loki that was far more magnanimous than mischievous.
Earth X and the Asgardians
Created by Alex Ross, Jim Krueger and John Paul Leon, the Earth X continuity was a dark future of the Marvel Universe, not unlike Mark Waid and Ross' Kingdom Come series for DC. Much like that series, it was eventually cemented as its own continuity, as many of the revelations in the story were incongruous with the mainstream universe. This world was one in which almost all of humanity had been mutated by the Inhumans' Terrigen Mists, which themselves had been released during a faulty experiment conducted by Reed Richards. Another heavily reimagined element was that of the Asgardians.
In this world, the Asgardians were not gods but instead aliens who had been manipulated by the Celestials. Due to their level of mutation, their very definition and appearance was determined by others within their environment, with the ensuing Norse legends willing the aliens into appearing to be the figures of myth. Odin was a one-eyed human who joined the group, manipulating his being into that of the All-Father. Loki was the result of "Odin's" willing the rest of Asgardian myth into existence, with much of Loki's life, including his hand in the Avengers coming together, the result of Odin's tampering.
In the present day of the Earth X world, Loki took on a form that was essentially a darker version of how he was usually depicted. His ambitions for conquest grew, and he even convinced Odin to turn Thor into a woman and have him taken out of the way, allowing Loki to freely enact his will on Earth. Before he could make anything of it, however, he was informed by X-51, the former Machine Man and the new Watcher, of the true origins of the Asgardian race. He attempted to convince the other Asgardians of the truth, unaware that Odin was behind the deception. After seemingly killing himself, Loki reemerged later in the series as a far more heroic figure.
Loki: The God of Thunder
In the story's sequels, Krueger, Ross and Doug Braithwaite's Universe X and Paradise X, Loki's transformation into a more heroic figure would eventually result in his most daring shapeshift of all. He deduced that it was Odin who was manipulating the Asgardians, and in an attempt to assert his identity for himself, he took on a featureless, literally neutral form. He saved the reborn Captain Mar-Vell, and tried to help Thor by convincing him of his control over his own identity. He then convinced the Absorbing Man to become vibranium to realign the world, proving that the former God of Mischief really had changed his ways.
This certified change of heart saw Thor working alongside his brother, making the once dark future a little brighter. Eventually, Thor reclaimed his human identity of Donald Blake, wanting to be of assistance to humanity in a more medical sense. Loki, however, felt that the world still needed a God of Thunder, and he himself had grown used to playing the role of a hero. Shapeshifting into Thor, complete with a hammer, "Loki" along with Ransak and Black Bolt became a new incarnation of the Avengers. This foreshadowed the more noble direction that Loki has taken in recent years, and predicted his eventually being able to pick up the hammer of Thor in the mainstream universe.