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'.
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.
Adding some new original characters can also be interesting. For example, a young dragon who looks up to Spyro and wants to help him during the Dark Days. This character could start out inexperienced but grow throughout the story, learning from Spyro and becoming an important ally. Or a mysterious wizard who has been observing the Dark Days from afar and decides to get involved, either to help or hinder Spyro's progress, adding an element of mystery and unpredictability to the fanfiction.
If it's a crossover with a Naruto - like ninja world, Naruto himself would be a great addition. His energy and determination could contrast and complement the World Trigger characters. Also, Sasuke with his powerful ninja skills could add an interesting dynamic. In a crossover with a One Piece world, Luffy would be a fun character to include. His carefree yet strong - willed nature would bring a new flavor to the story.
A witch could be an interesting character. She could be either an ally or an enemy to Dracula. Maybe she has her own motives in the vampire world and her magic could create all sorts of plot twists.
Natasha Romanoff would be an interesting character to include. She's very perceptive and could be the one to first notice the change in Loki's behavior towards Tony. And then there's Bruce Banner. His scientific mind could interact with both Loki's magic and Tony's technology, and he could also offer some unique perspectives on their relationship, perhaps warning Tony about the risks of getting involved with Loki.
The gods themselves could be interesting characters to include. For example, Poseidon could offer Percy more power or advice at crucial moments. Athena could also play a role, perhaps initially being an adversary because of her different goals, but later becoming an ally. Even minor gods like Hecate could be included. Her magic could be used to help Percy in situations where his normal powers aren't enough. Also, new characters created specifically for the fanfiction, like a powerful demigod from a previously unheard - of lineage, could add an element of surprise and freshness to the story.
The Dark Eldar Archon is an obvious choice. He is a powerful and cunning leader in the Dark Eldar society, full of ambition and a thirst for power. His complex motives and devious nature can drive a lot of the plot.
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'.
Zelda could be a great addition. She might initially be wary of Dark Link but then come to understand his situation. Navi could also be included. Her insights could help Link and Dark Link communicate better. And Ganon, of course. His presence could add more conflict. He might try to manipulate Dark Link for his own evil purposes.
Cynder could be a key character. She has a connection with Spyro and could play a role in helping Dark Spyro either come to terms with his powers or in fighting against a common enemy.