People write 'Vader protects Ahsoka' fanfictions because they are intrigued by the complex relationship between the two characters. Their history as master and apprentice, and then Vader's fall to the Dark Side, makes for a rich backdrop for stories of redemption and protection.
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.
Ahsoka can be developed by showing her growing understanding of Anakin and Padme's relationship. For example, she could start off being a bit naive about love and then gradually realize the depth of their connection. Also, her skills in the Force could be enhanced through her experiences with them. Maybe she has to face a powerful enemy alone while protecting Anakin and Padme, and this forces her to dig deeper into her Force abilities.
Naruto would probably be very curious about Ahsoka's Jedi powers. He might constantly ask her to show him how her lightsaber works. Ahsoka, on the other hand, would be interested in Naruto's ninjutsu and his ability to use chakra in different ways. They would likely start off with a bit of friendly competition, showing off their skills to each other.
Well, determining Ahsoka's age in the novel can be a bit tricky. It often depends on the timeline and events of that particular story. Sometimes it's explicitly mentioned, but other times you might have to do some detective work within the narrative to figure it out.
The Ahsoka novel is indeed canon. It fits within the established framework and narrative of the Star Wars universe and contributes to the overall lore.