At the end of Avengers, Loki seemed content to accept his punishment for his crimes against humanity, but what if he had kids after that time, and what if one of those kids decided to avenge his father's defeat and go after the Avengers?
It had been over twenty years since Loki had last seen the stars. Banished to a dark world, imprisoned, with his powers gone. He'd spent so long there he had nearly forgotten what Asgard looked like.
On that world though, Loki had found peace, and fallen in love with one of the Asgardians who worked there. He married her, and they raised their children on that world. A son and a daughter. Loki told them stories of Earth, and how he'd obtained a staff that controlled the tesseract, a powerful Asgardian artifact, and had sent an army to destroy the humans, yet been defeated by a team of misfits who called themselves the Avengers.
His children, Helena and Lucifer, listened raptly when he told him the stories, and hung onto every detail, always asking for more. They played games about the Avengers, and made up stories about them. The Avengers had become a symbol in their household.
When the children came of age, Loki was released and they moved back to Asgard, where they began a humble but pleasant life. Loki had given up any dreams of becoming king long ago.
But, somewhere, in the dark corners of his son's mind, a plan was forming. A dark plan of revenge...