A Sith apprentice could be a great antagonist. They might be after the same artifacts or information that the Padawan and Ahsoka are seeking. For example, if there's a powerful Sith holocron, the Sith apprentice would stop at nothing to get it.
One idea could be the Padawan's struggle with mastering new Force abilities. For example, they might have difficulty with telekinesis and keep accidentally dropping things. Another could be their adventures with Ahsoka on different planets, like facing a new type of alien threat on a jungle planet.
Obi - Wan Kenobi is also a potential character. A body swap with Ahsoka would lead to some comical moments as well as some deep character exploration. Obi - Wan would have to handle Ahsoka's more impulsive nature, and Ahsoka would get to experience Obi - Wan's calm and collected Jedi demeanor.
Perhaps a group of dark side cultists could be the main antagonists. They idolize the power of the dark side and see Ahsoka as a prime candidate to join their ranks. When she refuses at first, they start to hunt her down, but in the process, they unknowingly push her closer to the dark side, creating a complex dynamic where they are both the pursuers and the inadvertent cause of her fall to the dark side.
Her opponents could be the Separatist forces. Umbara was a planet involved in the Clone Wars, so it's likely that battle droids and their commanders would be her foes. They would be trying to gain control of Umbara's resources or strategic locations.
In some versions, it could be a former ally who has been corrupted. This character, once on Ahsoka's side, has now turned against her due to some external influence or personal greed. Their knowledge of Ahsoka's strengths and weaknesses would make them a formidable opponent, and they could be the one to cause her death.
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.
Another potential antagonist could be a jealous rival within Slytherin. This character might be envious of the 'Prince of Slytherin's status and popularity and would stop at nothing to bring him down. This could lead to a lot of internal Slytherin house drama and power struggles in the fanfic.
Most likely, the 'terror' itself is the antagonist. It's what the hero has to fight against.
One potential antagonist could be a rogue ninja who also specializes in bojutsu. He might be jealous of the main character's skills and tries to undermine him at every turn. Another option is a powerful villain from the Naruto universe who sees the bojutsu master as a threat to his plans.
One possible plot could be a story where Darth Vader, in his full potential, has a moment of self - realization during a battle against Ahsoka. He might start to question his dark path and Ahsoka, seeing this glimmer of Anakin in him, tries to pull him back to the light. It could be a complex emotional journey for both characters as they grapple with their past relationship as master and apprentice.