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.
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.
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.
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.
I'm not sure specifically about Loki x Thanos mpreg fanfiction. Fanfiction can cover a vast range of pairings and themes, but this particular one might be quite rare or not very mainstream. You could try searching on some large fanfiction platforms like Archive of Our Own to see if there are any works of this kind.
There might be a moment where Loki discovers a crucial piece of information about Thanos. For example, he finds out the true source of Thanos' power and figures out how to cut it off. This discovery could be a turning point in the fanfiction.
Power is a common theme. Both Thanos and Loki are power - hungry in their own ways. In fanfictions, they often either compete for power or try to use each other's power for their own goals. Another theme is redemption. Sometimes Loki is seen as a way for Thanos to find redemption, or vice versa.