One possible plot could be that they get the curse marks from a new, powerful enemy. They then have to figure out how to control the curse marks together without losing themselves to the dark power. Maybe they go on a journey to find a way to remove the marks while also facing challenges from other villages who see them as a threat because of the marks.
You can try some popular fanfic websites like Fanfiction.net or Archive of Our Own. Just search for 'Naruto Sasuke curse mark' on their search bars, and you might find some relevant fanfics.
The curse mark can change their physical abilities. For Sasuke, it might enhance his speed and strength, but also make him more prone to losing control. Naruto, on the other hand, might develop new chakra - related powers because of his exposure to Sasuke's curse mark. For example, he could gain the ability to sense the curse mark's energy better.
In some fanfictions, the curse mark can initially drive a wedge between Naruto and Sasuke. Sasuke might become more withdrawn and brooding due to the power and dark influence of the curse mark. Naruto, on the other hand, would be worried about his friend but might not fully understand what Sasuke is going through. This could lead to some misunderstandings and arguments between them as Naruto tries to reach out while Sasuke pushes him away.
Their friends might be initially afraid of them. Since the curse mark is often associated with dark power, friends like Sakura and Kakashi could be worried that Naruto and Sasuke might lose control. This could create some distance between them as their friends are not sure how to approach them anymore.
Naruto's relationship with Sasuke could be really affected. Sasuke, having his own experiences with dark powers, might see Naruto in a new light. They could either become more competitive as they both struggle with different forms of dark power, or they could bond over their shared experiences of having something that sets them apart from others.
One interesting plot could be how the curse mark affects their relationship. Maybe Sasuke's curse mark starts to make him more aggressive towards Naruto, but Naruto is determined to save him from its dark influence. This could lead to some intense battles and emotional moments between them.
Another theme is identity. Characters with the curse mark may feel different from others, and the fanfic often explores how they come to terms with this new part of themselves. For example, they might question if they are still the same person they were before getting the mark. It's a complex exploration of self in the context of the Naruto world.
The curse mark could make Naruto more cautious. Since it's a dangerous and unpredictable power, he might think twice before acting impulsively as he usually does. For instance, in battle, instead of rushing in head - first, he'll consider the consequences of using the curse mark's power.
Well, a possible plotline is that the curse mark is linked to an ancient and dark power in the Naruto world. Naruto, with the curse mark, starts to have visions or dreams related to this power. As he tries to figure out what it all means, he also has to deal with the physical and mental toll the curse mark is taking on him. His training then becomes not only about getting stronger in general but also about mastering this new and dangerous aspect of himself.