In this fanfiction, perhaps Thor hits Loki with Mjolnir as a form of punishment for one of Loki's many tricks. Maybe Loki has double - crossed Thor in a plan to take over Asgard, and Thor's only option to stop him is to use his powerful hammer.
The fanfiction might start with Loki causing chaos in Asgard once again. Thor, having had enough of his brother's antics, decides to use Mjolnir against him. This could be a very emotional moment as Thor may be conflicted about using such force against his own kin. But Loki's actions may have put Asgard in such great danger that Thor feels he has no other choice. This event could then set off a chain of events where Loki is either redeemed or becomes an even greater foe.
Well, it could be that Loki has angered Thor in some way that goes beyond their usual bickering. The act of Thor hitting Loki with Mjolnir might be a turning point in their relationship. It could lead to Loki realizing the error of his ways, or it could drive an even deeper wedge between them, with Loki vowing revenge for such a public and powerful blow.
It could be argued that Loki is also a kind of protagonist in his own way. Even though he is on the receiving end of Thor's hammer, his actions leading up to that moment are likely what drive the story. His schemes and plots are what cause Thor to react, so in a sense, he is a central figure in the narrative as well.
The plot could involve Thor being led to Loki's tree by a vision. He arrives there and discovers that the tree has some sort of magic that is tied to Loki's past or future. Maybe it holds the key to saving Loki from a great danger.
Maybe Loki is betrayed by someone he thought was a friend, and he is deeply hurt both physically and mentally. Thor discovers what has happened. He becomes extremely protective. He first takes care of Loki's wounds, using his own knowledge of healing herbs. Then he sets out to find the one who betrayed Loki. Along the way, Thor makes sure Loki feels safe and loved, constantly reassuring him that he will always be there for him. This journey not only heals Loki's physical and mental wounds but also deepens their brotherly bond.
Well, one possible plot could be that Thor and Loki's little sister has some special powers that are a mix of their own. She might be caught in the middle of their feuds, trying to bring peace between them. For example, she could use her powers to stop Loki from his mischievous plans that would anger Thor, while also making Thor understand Loki's motives better.
Well, perhaps Loki freezes Thor as part of a deal with an enemy. Thor is frozen and the enemy thinks they can now easily take over Asgard. But Loki has a hidden agenda. He freezes Thor not to harm him but to protect him from a more dangerous plot. Thor, while frozen, has visions or dreams that give him some new power or knowledge. When Loki finally unfreezes him, Thor is stronger and together they outwit the enemy and Loki's true loyalty is revealed to Thor, strengthening their bond.
One possible plot could be that Laufey, Loki's biological father, decides to use Loki against Thor. He might abuse Loki in some way, either physically or emotionally, to make Loki do his bidding, which could involve causing trouble for Thor. Loki, being in a difficult position, has to deal with his complex feelings for Thor while also trying to escape Laufey's control.
A plot development could be Loki forming an alliance with the Jotuns against a common enemy. This enemy could be a threat to both Jotunheim and Asgard. Loki, being in Jotunheim, has the opportunity to broker this alliance, which would surprise Thor and the other Asgardians when they find out.
It could be a very emotional reunion. Thor might be initially angry at Loki for all his past misdeeds but then quickly soften as he realizes how much he has missed his brother. Loki, on the other hand, might be a bit guarded at first, still not fully trusting Thor's reaction. But eventually, they would embrace and start to rebuild their relationship.
In the fanfiction, Thor could be a key factor in Loki's development within the TVA. Since Thor has always had a strong influence on Loki, his presence might make Loki question his actions and motives even more deeply. For example, Thor could remind Loki of their past, both the good and the bad times, which could lead Loki to make different choices within the TVA's framework.
There's 'Thor's Redemption in Loki's Arms'. In this story, Thor realizes his true feelings for Loki after a series of events that nearly tear Asgard apart. It's a tale full of emotion and character development.