She might start wearing darker colors. Instead of her usual white and orange, she could don black robes with red accents, symbolizing her new allegiance to the dark side.
Her overall posture could change to be more menacing. She would stand taller, with a more confident and intimidating gait. Her facial expressions would be more severe, lacking the warmth and compassion she once had. Her lightsabers could also change in appearance, perhaps with a more jagged blade design or a red - tinted glow to signify the dark side energy flowing through them.
Her lekku could take on a more menacing look. Maybe they become a bit darker in color, or she could adorn them with dark side symbols or spikes. Her eyes might also change, having a red glow or a more intense, cold look.
She would become more ruthless. Instead of her usual calm and collected self, she'd be quick to anger and more likely to use force as a first option rather than a last resort. For example, she might not hesitate to harm innocents if they got in the way of her goals.
Well, at first, they might seem closer as they are now both part of the same 'dark' world. But soon, the dark side's influence of greed and power - hunger would start to drive a wedge between them. Anakin, with his already tumultuous relationship with power, might try to dominate Ahsoka. Ahsoka, having been dragged into the dark side against her will perhaps, would resent Anakin for it. Their relationship would be a volatile mix of shared darkness and individual power struggles.
Her relationship with Anakin would be severely strained. Anakin, as a Jedi (at least initially in this scenario), would be horrified to see her turn to the Dark Side. He might try to bring her back, but there would be a great divide between them. Their former master - apprentice bond would be tested, and it could turn into a bitter rivalry as Anakin tries to uphold the Jedi way and Ahsoka becomes more entrenched in the Dark Side.
One possibility in fanfiction could be that Ahsoka is tricked into the Dark Side. Maybe an old enemy manipulates events to make her feel betrayed by the Jedi Order. This could lead her to explore the power of the Dark Side, believing it to be a way to seek justice on her own terms.
The influence of a powerful dark side user who targeted her specifically. This user could manipulate her emotions, play on her insecurities and gradually turn her towards the dark side. They might offer her knowledge or power that she couldn't resist, slowly corrupting her view of the Force.
Maybe Ahsoka gets captured by a dark side cult. They try to convert her by showing her their version of 'justice' through the dark side. They expose her to their cruel and efficient ways of dealing with enemies, which makes her question her Jedi teachings. However, she eventually realizes that their 'justice' is just tyranny in disguise.
She might be tempted by the Dark Side if she discovers a hidden truth about the Jedi Order that she can't accept. So she turns to the Dark Side thinking it's a better alternative. For instance, if she finds out that the Jedi have been hiding some dark secrets about the Force, it could push her over the edge.
One possible plotline could be that Ahsoka feels betrayed by the Jedi Order. Maybe she discovers some dark secrets within the Order that make her lose faith in the light side. This could lead her to be lured by a powerful Sith Lord who offers her a new way of seeing things, and slowly she starts to embrace the Dark Side, using her skills in new and more aggressive ways.
If Ahsoka turns to the dark side, it would be a huge blow to the morale of those who still believe in the light side. She was a symbol of hope and a skilled Jedi, so her fall would make many question the strength of the light side. It could also lead to more chaos as she might use her skills to cause destruction or help the dark side in their plans.
One aspect of Ahsoka in dark side fanfiction might be her change in appearance. She could start wearing darker robes and her lekku might even seem to have a more menacing look. Her fighting style would also likely change, becoming more aggressive and less about defense. Writers often explore how her relationships with other characters, like Anakin, would be affected. Anakin might be torn between trying to save her and seeing her as a threat.