One possibility is that Voldemort wants to break Harry's spirit. By putting him in timeout, he isolates Harry from his friends and allies. Harry, being the strong - willed character he is, could use this time to gain some inner strength or discover a new magical ability that will help him when he finally faces Voldemort again.
In this fanfic, perhaps Voldemort has some grand plan. He puts Harry in timeout to wait for the right moment. He might be using this time to gather his Death Eaters and prepare for a big showdown. Harry, while in timeout, could be thinking of ways to break free and defeat Voldemort once and for all.
Hermione could be a main character. She might be trying to find a way to rescue Harry from the timeout situation. Ron could also be there, supporting Hermione in her efforts.
One possible plot could be that Harry tries to reason with Voldemort about the futility of his dark ways. He might talk about how much destruction Voldemort has caused and how many lives have been lost. Harry could also try to appeal to any remaining shred of humanity in Voldemort, perhaps by bringing up his own orphan status and how they both lost their parents in a way, though for very different reasons.
Maybe Harry has all of Voldemort's powers but uses them for good. He could start to look like Voldemort but with a kind heart, and he uses his new - found dark magic to fight off new threats that are even more powerful than Voldemort ever was.
In the 'harry joins voldemort fanfic', it could start with Harry being captured by Voldemort. Instead of killing him immediately, Voldemort decides to brainwash him. Through a series of dark magic spells and psychological manipulation, Harry starts to see Voldemort as a mentor. As time passes, Harry becomes an integral part of Voldemort's inner circle, helping him plan attacks against the wizarding world, believing that they are actually for a greater good as Voldemort has convinced him.
Well, one possibility could be that Voldemort adopts Harry as a way to control the wizarding world. He might think that by having Harry on his side, the resistance against him will crumble. Another could be that Voldemort discovers some sort of connection between him and Harry that makes him want to take Harry under his wing, perhaps related to some long - forgotten prophecy.
Well, it could involve Voldemort having some strange newfound attraction to Harry and trying to make Harry his 'queen' in a very dark and twisted way. Maybe he uses dark magic to bind Harry to him and Harry has to find a way to break free.
In such a fanfic, the plot could involve Harry finding a long - lost magical artifact that reveals his connection to Voldemort as his heir. As a result, he starts to have strange dreams and visions similar to what Voldemort had. The wizarding world would be in chaos as they try to figure out if Harry is going to turn to the dark side. Harry himself would be in a moral dilemma, trying to decide whether to embrace this new identity or fight against it.
Perhaps Voldemort discovers that there is a greater threat out there. In the fanfic, it could be that another dark force is emerging, and Voldemort sees Harry as an ally of sorts against this new threat. So, he decides to protect Harry in order to deal with this common enemy.
Well, one possible plot could be that Harry is somehow transformed into the snake against his will. He then has to figure out how to break the spell while being in the form of the snake and also being close to Voldemort. He might use his knowledge of Voldemort's secrets to his advantage even in that form.
In this fanfic, Harry might be betrayed by the Order of the Phoenix. They could have made a decision that endangered Harry's life without his knowledge. Feeling used and discarded, Harry runs away. Voldemort, through his Death Eaters, finds Harry and manipulates him. He tells Harry that he can give him a place where he will be respected and not betrayed again. Harry, still young and naive in some ways, falls for Voldemort's lies and joins him, leading to a very different kind of war in the wizarding world.