In the Star Wars universe, such a scenario would be highly complex and out of the ordinary. Ahsoka has a deep connection with the Jedi teachings, and Darth Vader is on the dark side. If she were pregnant, it would likely be a situation full of moral and ethical dilemmas. She might struggle with protecting the child from the Sith and Vader's influence, and also deal with how the Jedi Order would react, as they have strict rules about relationships and attachments.
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.
They might face the challenge of raising a child in a galaxy that is often at war. Finding a safe place to live and provide for the child would be difficult. Also, they have different heritages. Rex is a clone, and Ahsoka is a Togruta. Their child might face discrimination or confusion about their identity. They would need to figure out how to teach the child about both of their cultures.
Anakin Skywalker is a popular choice. Their relationship as master and apprentice in the Star Wars universe provides a lot of material for fanfics. Another popular character is Rex, as they have a history of fighting side by side in the Clone Wars.
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.
Sabine Wren is often featured alongside Ahsoka. Sabine is a Mandalorian warrior with her own set of skills and a complex personality. She and Ahsoka can have great interactions, whether it's in combat or during strategic planning. Another common character is Ezra Bridger. He has his own connection to the Force and his adventures often cross paths with Ahsoka's, creating interesting storylines.
In Clone Wars Ahsoka fanfiction, Padmé Amidala could be a great character to pair with Ahsoka. Padmé was involved in the political side of the Clone Wars, and Ahsoka was on the front lines. Their interactions could explore the different aspects of the war. Another interesting character would be Ventress. Their battles in the Clone Wars were intense, and in a fanfiction, they could form an unlikely alliance or have more complex interactions beyond just fighting each other.