It could be a dark wizard that was previously unknown. Maybe this wizard has a grudge against Harry's family for some long - forgotten reason and is using a very subtle and complex plan to harm Harry. And Fred discovers this wizard's plot when he goes back in time.
One popular fanfiction with this plot is 'Time's Redemption'. In it, Hermione uses a powerful time - turning device to go back. She faces many challenges in the past, like avoiding detection by other characters while trying to figure out how to prevent Fred's death. It's a great read for those who love emotional and action - packed stories.
Well, perhaps when Fred goes back, he discovers that the danger to Harry is much more complex than he initially thought. There could be a traitor among the people he thought were allies in the past. Also, Fred himself might get stuck in the past and has to find a way to get back while still ensuring Harry's safety. Maybe the time - traveling magic is being manipulated by a hidden enemy, and Fred has to figure out who and how to stop it to save Harry.
She could accidentally change other things in the past. For example, her presence might disrupt some other events that were supposed to happen. Maybe she prevents another character from getting a crucial piece of information, which then has a ripple effect on the future.
One great fanfiction with this plot is 'Time's Redemption'. In it, Harry uses a powerful time - turning spell he discovers in an ancient book. He has to face many challenges in the past, like avoiding detection from the Ministry's time - turner regulations. Another good one is 'Back to Save Sirius'. Here, Harry teams up with Hermione and they use a secret time - travel device left by Dumbledore.
One possible plot twist could be that when Harry goes back, he discovers that Snape's situation is far more complex than he initially thought. Maybe Snape was involved in some secret mission that put him in danger, and Harry's interference actually endangers the entire wizarding world in unexpected ways. For example, Snape might be on the verge of uncovering a new dark magic plot that Harry's presence disrupts.
One possible plot twist could be that Snape, in his attempt to save Harry, accidentally changes the entire future of the wizarding world. For example, he might prevent Voldemort from ever rising to power in the first place, which would mean that all the relationships and events that we know from the original story would be completely different. Another plot twist could be that Snape discovers that saving Harry is not as straightforward as he thought. Maybe there are dark forces at work that are trying to stop him from changing the past, and he has to fight against them while also trying to protect Harry.
He might use some powerful magic artifacts. For example, the Time - Turner, if he can get his hands on an even more powerful or modified version of it. This could allow him to bypass some of the normal time - travel restrictions.
Well, it could be that Sonic goes back in time because Shadow is in a dire situation that only Sonic can prevent. This could be due to a threat from a powerful villain that managed to overpower Shadow in the past. Sonic, being the hero he is, decides to take the risk of time travel. He might have to use some sort of time - traveling device or power. Once in the past, he'll need to quickly figure out how to stop the threat before it's too late. This fanfiction idea is really cool as it combines the action - packed nature of Sonic's world with the concept of time travel.
One common plot point is the discovery of a way to time - travel. It could be a new spell, a hidden device, or something left by a powerful wizard. Another is the struggle to avoid changing too much of the past. Harry usually has to be very careful not to disrupt the timeline while trying to save Sirius. For example, he can't accidentally stop something important from happening that is related to the future.