Ichigo might gain the ability to manipulate shadows with the Darkness Zanpakuto. He could use the shadows to hide, to attack from unexpected angles, or even to create illusions. This would give him a whole new set of tactical options in battles. He could also absorb the darkness around him to strengthen his attacks or heal himself, much like how he currently absorbs spiritual energy.
The Darkness Zanpakuto could also grant Ichigo the power to corrupt or influence the minds of his enemies. He might be able to plant seeds of doubt or fear in their minds, making them easier to defeat. However, using such powers could also have a negative impact on Ichigo himself, as he has to fight against the dark nature of the Zanpakuto to maintain his own sanity and moral compass.
The fanfiction could also explore how Ichigo's relationships change due to his Darkness Zanpakuto. His friends might be scared or distrustful of him. Rukia, for instance, could be torn between helping Ichigo master this new and dangerous power and being worried about the consequences it might have on him and those around him.
It could be a fanfiction that delves into the power dynamics. Since Ichigo has three zanpakuto, perhaps it shows how his power has increased exponentially. It might also be about the training he had to go through to be able to handle three zanpakuto, and how this changes his role in the Bleach universe.
One idea could be that the female Zanpakuto has a unique personality. For example, she could be sassy and constantly tease Ichigo while also being fiercely protective of him. In the fanfiction, she might have her own backstory, like being created from an ancient power that was sealed away for centuries until Ichigo awakened her.
His friends might be in awe. After Ichigo meets his real Zanpakuto, his friends could be both amazed and a little bit intimidated by the new power he has. They would still support him, but there could be a new sense of respect and a bit of distance as they realize how much stronger he has become.
Another way Ichigo could get his powers back in fanfiction is through a magical or spiritual artifact. Maybe he discovers an ancient sword or a talisman that has the ability to restore his lost powers. This could be part of a grander plot where he has to go on a quest to find this item, facing various challenges and enemies along the way. During this journey, he might also learn more about the true nature of his powers and how they are connected to the overall 'Bleach' universe.
Ichigo's powers would adapt by evolving through new challenges. In Fairy Tail, he'd face different kinds of enemies compared to Bleach. His ability to sense spiritual pressure might be tweaked to sense magic power instead. His Hollow powers, which are often seen as a dark side in Bleach, could be seen as a powerful and somewhat unruly form of magic in Fairy Tail. He would need to learn to control it better in the new environment.
His powers could be more nature - based. For example, instead of just controlling water, he might be able to control all forms of liquid in nature, like sap or blood. His connection to the wild could also make him more attuned to the emotions of animals, allowing him to communicate with them more effectively.
Well, in a fanfiction where Percy is born in Greece, his powers would likely have a more 'Greek - flavoured' aspect. Since he's in the homeland of the Greek gods, his connection to water could be intertwined with the Greek concept of the four elements. So, his water powers might interact more with the other elements in a way that is more in line with Greek elemental theory. He might also be able to draw on the power of Greek water deities more directly, which could enhance his abilities and give him new ways to use his powers in battles and other situations.
His spider - sense might be enhanced. Since Athena is known for her wisdom and foresight, his ability to sense danger could be more refined, allowing him to predict attacks from enemies even more accurately.
Well, he'd probably have a very different fighting style. The Arrancar Zanpakuto might have unique abilities that are more in line with Hollow powers. He could potentially have enhanced speed and strength, and the release form might be something really wild, like a combination of his Shinigami and Hollow forms.