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.
Perhaps in a more comical or crack - fic type of plot, Loki uses his legs in a dance - off against Thanos to win his freedom or something. And during this dance - off, Thanos starts to see Loki in a new light and feelings develop. This could be a really wild and wacky take on the pairing and the 'legs' aspect.
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.
Perhaps the main antagonist could be Loki himself, at least in the beginning. Since he got involved with Thanos' legs in some improper way, Thanos might see him as the antagonist. But as the story progresses, they might find a common enemy. So, initially, Loki's actions and motives that revolve around the legs could make him the antagonist in Thanos' eyes.
You might try fanfiction websites like Archive of Our Own (AO3). They have a large collection of fan - created works. Just use the search function and type in 'Thanos x Loki' and see if there are any stories related to the specific 'legs' aspect you're interested in.
In a Loki x Thanos mpreg fanfiction, a potential plotline could be that the pregnancy is part of a prophecy. Loki and Thanos have to deal with the implications of this prophecy, whether it means the child will bring great power or destruction. There could be a lot of internal struggle for Loki as he contemplates his role as a soon - to - be parent, especially considering Thanos' nature. And Thanos could be using the situation to gain an advantage over his enemies, all while trying to protect Loki and the unborn from those who would oppose them.
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.
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.
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.
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.
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.
Well, one possibility could be that Tony sacrifices himself in a final battle against Thanos. Maybe he uses some new, untested technology that ultimately drains his life force but also manages to defeat Thanos. Another idea could be that Thanos, in a moment of unexpected mercy, spares Tony's life initially but then something goes wrong in the aftermath, leading to Tony's death. For example, perhaps the Infinity Stones' power backlash affects Tony in a way that kills him.