In fanfiction, Loki's time with Thanos could involve Loki trying to outwit Thanos. Maybe he's forced to do Thanos' bidding at first but secretly plans to undermine him. He could be gathering information about Thanos' plans and weaknesses.
Well, one possible plot could be that Thanos uses his mind - controlling powers on Loki to make him steal powerful artifacts for his collection. Loki, being under Thanos' control, has to go against his own nature at times. For example, he might have to target Asgardian relics that he would otherwise protect.
Well, perhaps in this fanfiction, Loki is cursed to be attached to Thanos' legs for a period of time. This forces them to work together in a very strange way. Loki tries to use his wits to find a way to break the curse, while Thanos is frustrated by Loki's constant attempts at trickery. Along the way, they might encounter other characters from the Marvel universe who are both amused and confused by their situation.
One possible plot could be that Thanos sees Loki as a tool that has failed him. So, he subjects Loki to various forms of physical and mental abuse to break his spirit. Loki, being a survivor, tries to find ways to outwit Thanos even in his weakened state.
Well, it could be something like Thanos becomes intrigued by Loki's legs for some strange, otherworldly reason. Maybe Loki has some sort of magic or power that is centered around his legs that Thanos wants to possess or control.
In this fanfic, Thanos could be using Loki as a pawn in a much larger scheme. Loki, who is now controlled, might be used to start a war between different realms. However, Loki's own cunning nature and his relationships, like his complex relationship with Thor, could come into play. Thor might notice that something is off with Loki and try to break Thanos' control over him. This could lead to a series of events where Loki slowly regains his free will and turns against Thanos in the end.
Another scenario could be that Loki forms an unexpected alliance while in captivity. He could meet some other captives who have a grudge against Thanos. Together, they could plot against Thanos from the inside. Loki's silver tongue might be able to convince these captives to trust him and follow his lead. They could create a distraction or a mutiny within Thanos' forces, which would give Loki an opportunity to either escape or turn the tables on Thanos.
Another possibility is that Loki teams up with other characters. He could form an alliance with the Avengers or some other powerful beings in the universe. Together, they could come up with a plan to stop Thanos. Loki could contribute his unique skills, like his ability to shapeshift and infiltrate Thanos' ranks, and then they could launch a coordinated attack.
Loki could also show signs of fear, though he would try to hide it. Inside, he knows the power that Thanos wields. But on the outside, he would maintain a facade of bravado. His reactions would be a mix of his natural cunning and the instinct to survive in a very dangerous situation.
Loki may become more cunning. His time with Thanos, a very powerful and dangerous being, forces him to be on his toes all the time. He might learn new ways to deceive and manipulate, which are skills he already has but are sharpened during this time.
Well, Loki might become more loyal to Thor. After being saved from Thanos, he could see Thor in a new light. Maybe he would stop his mischievous ways for a while and actually help Thor in his future battles. He might even start to respect Thor's leadership more.