In a fanfiction battle between Ahsoka and Vader, it's really hard to say who would win. Ahsoka is highly skilled and has a deep connection to the Force. But Vader is extremely powerful with his Sith abilities. It could go either way depending on how the writer wants to portray it in the fanfiction.
One of the most dramatic moments could be when Ahsoka first realizes she's facing Vader, knowing he was once Anakin. It's a moment full of shock and disbelief. Another could be when they clash lightsabers for the first time, the sparks flying as the power of the Force is felt around them.
A really exciting moment could be when Ahsoka and Vader engage in a battle of wits as well as a physical battle. For example, Ahsoka might try to appeal to the Anakin still inside Vader, which could cause a moment of hesitation. Or Vader could taunt Ahsoka with memories of her past failures. The build - up to the final blow, whether it's Ahsoka's strike or Vader's, with all the emotions and power on display, would also be an extremely exciting moment in a fanfic.
In a fanfic, it's really up to the writer. However, considering their abilities, Vader is extremely powerful with the Force, but Ahsoka is very agile and has unique fighting techniques. If Ahsoka can use her speed and agility to her advantage, she might be able to outmaneuver Vader. But Vader's brute strength and experience in combat are also not to be underestimated.
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.