One idea could be a fanfiction where Loki uses his magic to cover up his own dark circles. Maybe he's been up all night plotting against Thor and is trying to look his best in front of the other Asgardians. He could accidentally create a spell that gives everyone in the palace dark circles instead.
Maybe Loki pretends to be a mortal female to infiltrate a powerful wizard's lair. He has heard that the wizard has a weakness for mortal women. As a female, Loki is able to charm his way into the lair. Once inside, he discovers that the wizard is planning to use a powerful spell to take over all of Asgard. Loki then has to figure out a way to stop the wizard while still maintaining his female disguise.
One idea could be a story where Loki teams up with the Winchester sister to battle a common supernatural foe. Maybe they start off distrusting each other, but as they face challenges, they learn to work together. Loki's trickery could be both an asset and a source of conflict in their partnership.
One idea could be that Loki, being the mischievous character he is, finds his way into the wizarding world of Harry Potter. He could start causing chaos in Hogwarts, perhaps by disguising himself as a student or a professor and playing pranks that are far more elaborate than those of the Weasley twins. For example, he might turn the entire Great Hall into an ice rink during breakfast time.
One idea could be that Loki gets turned into a vampire while in the midst of his schemes against the Avengers. This new state gives him different powers and motives. He might struggle with his newfound thirst and also use his vampire abilities to outwit the Avengers in different ways.
Imagine a Loki OC who is a half - Asgardian, half - Vanir orphan. This OC has been living on the fringes of society, but Loki discovers their existence and decides to use them for his own schemes at first. However, the OC's pure heart and kind nature start to change Loki's perspective, and they end up on an adventure together to uncover a long - lost secret about their shared heritage.
One idea could be a crossover where Fenrir from the Harry Potter universe somehow gets transported to the realm of Loki. Loki, being the trickster, might see Fenrir as a tool for his own schemes. Harry Potter, perhaps in a quest to stop some dark magic related to Fenrir, ends up following him and has to deal with Loki's antics as well.
A unique Loki Marvel fanfiction idea could be Loki getting trapped in a time loop. Every day he has to relive the same events but tries to find a way to break the loop and change his fate. Or perhaps a story where Loki has to pretend to be another popular Marvel hero and hilarity ensues as he tries to act like them. Another concept is Loki forming an alliance with the villains of the Marvel universe but with his own hidden motives.
One idea could be Loki getting angry at Thor for constantly overshadowing him in Asgardian affairs. Maybe he feels that Thor is always getting the glory while he is left with the scraps. Another could be Loki being angry at the Avengers for imprisoning him after the New York incident. He might plot elaborate revenge in the fanfic.
One idea could be that in the fanfiction, Fem Harry accidentally stumbles upon Loki near the Purity Pool. Maybe she's seeking solitude and finds Loki there first. Loki, being the mischievous character he is, could initially be distrustful of her but then realizes she has no ill intent. They could start a conversation about the magic or special properties of the Purity Pool.