It could be a new and powerful dark side user. This character might have emerged from the shadows, having trained in secret, and their sole purpose is to take down Anakin as he is seen as a threat to their plans for galactic domination.
Perhaps a group of bounty hunters could be the ones to kill him. They could be hired by someone who wants Anakin out of the way. The bounty hunters would use their combined skills and resources to track Anakin down and finally eliminate him.
In some fanfictions, it could be an old enemy that Anakin thought he had defeated long ago. This enemy could have been biding their time, gathering strength, and finally gets the chance to take revenge on Anakin by killing him.
One possible scenario could be that he is killed in a lightsaber duel by a powerful Sith opponent. Maybe this new Sith has some secret technique that Anakin couldn't counter.
Yoda would be another character. As one of the wisest Jedi, he could sense the strange situation with Anakin and offer his unique perspective. He might be more patient with a child - like Anakin, teaching him important Jedi lessons and trying to ensure that Anakin doesn't go down the same dark path as he did in the original timeline.
In the fanfiction, Anakin may use his sense of humor to cheer Ahsoka up. He could make a light - hearted joke about the situation. For example, if she's upset about a mission gone wrong, he might say 'Well, at least we learned what not to do next time, right? And that's a victory in itself!' This would make Ahsoka see the positive side and feel better.
It could be the Separatists. They might see Anakin as a valuable hostage to use against the Jedi and the Republic. His skills as a Jedi Knight make him a prime target for them to gain some sort of leverage in the war.
If Anakin killed Obi Wan, it could have a domino effect. Anakin's relationship with other characters would likely change. Maybe the Jedi Order would crumble faster, and the Sith would gain more dominance. It's a fascinating 'what if' scenario to explore in fanfiction.
It could have completely changed the course of the Star Wars universe. With Palpatine out of the picture, the Sith might have crumbled and the Jedi could have regained their influence.
The Padawan could be a young alien from a species not often seen in the Jedi order. Maybe an amphibious species that has a different way of sensing the Force due to their unique physiology. This would make for an interesting dynamic as Anakin would have to adapt his teaching methods.
One possible plotline could be Anakin having to re - learn the ways of the Force as a child while also dealing with his past memories. Another might be how the other Jedi react to this de - aged Anakin, whether with suspicion or a sense of protectiveness.
There are also fanfictions where Anakin gets hurt in a ship crash. The ship could be attacked by enemy forces, and when it crashes, Anakin suffers various injuries like a head wound and broken ribs.
One possible plot development could be that his friends immediately launch a rescue mission. They might use their unique skills and resources to track down the kidnappers. For example, if Obi - Wan is involved, he could use his Jedi mind tricks to interrogate possible informants in the underworld.