What, in my mind, could have happened that explains the scene in Thor 2 when Loki revealed himself as being Odin at the end of the movie. (Written to be as brief as possible with no less than 400 words)
He smiled with great mirth as he watched his brother leave the throne room. He has once again deceived the dullard of an Asgardian. As he rejoiced at the thought, his face formed a sinister look. The façade that kept him from being recognized slowly faded and as the shroud that is the image of Odin withdrew, Loki revealed his true self.
An Asgardian guard reported back to the all-father telling him that they had found a body.
"My liege, I have come bearing terrible news. Loki is dead." The man then proceeded to rise from his genuflected stance.
"And the cause?" Questioned Odin, having what seemed to be genuine concern plastered over his face.
"Odin, sire… From what Thor has told us, Loki fell in the hands of the corrupted dark elf. He has done so while attempting to save his brother and the woman from Midgard." The guard then gave a sly smile that was too quick for the all-father to catch.
Odin stood up from his throne and ordered all in the throne room to vacate the area. As everyone walked out, the guard then leapt in the air and took several daggers from extra dimensional pockets that were hidden by a spell of some sort. "What is the meaning of this? This is treason!" Odin bellowed with great anger. But it was too late for the doors have been locked and had placed upon them an enchantment.
"You know very well what this means, father!" Revealing his true self, Loki came at his father with blinding speed and incapacitated him, then, he placed an energy field around Odin and initiated the Odinsleep. He cast an enchantment to conceal the all father from all forms of detection, even that of Heimdall's.
Loki then took on his adoptive father's form and proceeded to rule as King of Agard. With a sarcastic tone he said "Long live the king!" and as if on schedule, Thor came in to seek an audience with his 'father' and told him all about Loki's heroism (much to the god of mischief's delight) and the events that took place in Midgard.