Definitely Thor. Their complex relationship always adds depth to any story. You can explore their brotherly bond, rivalry, and how it changes in the context of 'The Dark World'.
One idea could be Loki having to team up with an unexpected ally from Asgard's underworld to stop a new threat that emerged during the events of 'The Dark World'.
The relationship between Loki and Thor is also frequently explored. In the Dark World, there's a lot of tension and history between them. Fanfictions might show them reconciling, or further straining their relationship, and how this affects the overall story of the Dark World.
Sure. One great Loki Dark World fanfiction is 'Loki's Redemption in the Dark World'. It delves into Loki's internal struggle during the events of The Dark World, exploring his complex relationship with Thor and his own motives in a really interesting way.
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.
Another plot could be a what - if scenario. What if Loki didn't fall into the abyss in The Dark World? He could have used his trickery to manipulate the dark elves in a different way. He might pretend to side with them at first to gain their trust and then double - cross them to save Thor and Asgard. This would show his complex nature and his loyalty to his home, in his own twisted way.
In a fanfiction, Loki could form an unlikely alliance with the Grandmaster. Maybe they both have a common enemy, like a powerful intergalactic force. Loki, being the sly character he is, and the Grandmaster, with all his resources, could team up. Their interactions would be full of banter, with Loki constantly trying to manipulate the situation and the Grandmaster trying to keep Loki in line while still using his skills.
A plot twist could be that Loki's dark circles are actually a sign of a hidden power. In the fanfiction, as Loki discovers this, he has to learn to control this new aspect of himself. It could be that the darker the circles, the more powerful he becomes, but he also risks losing his sanity if he doesn't master it.
Dark Loki Fanfiction can be appealing because it often delves deeper into the psychological aspects of Loki. We get to see his inner turmoil, his possible descent into darker paths. It can also create more intense and complex relationships with other characters. For instance, his interactions with Thor or other Asgardians might be completely different in a 'dark' scenario, which makes for a very interesting read.
Power dynamics is also a common theme. Loki has his powerful and often dark magic, and Jane is just a mortal. How they interact in terms of power, with Loki being so much more powerful yet being affected by Jane in some way, is something that many 'dark loki and jane fanfiction' explore. For example, Loki may use his power to protect Jane in his own dark way, or his power may accidentally harm her, leading to conflict and growth in their relationship.