Well, imagine the Susanoo as a giant warrior, and with the Rinnegan, Naruto (or whoever has these powers) could enhance it in multiple ways. He could use the Rinnegan's Preta Path to absorb chakra - based attacks while the Susanoo is attacking. The Rinnegan's Deva Path could also be used to repel enemies that get too close to the Susanoo. And the Susanoo's weapons could be imbued with the power of the Rinnegan, like a sword that can cut through any defense.
One idea could be a story where Naruto somehow acquires the Rinnegan and combines its powers with the Susanoo. Maybe he has to face an extremely powerful enemy that can only be defeated by this new, super - powered combination. He could start by accidentally awakening the Rinnegan during a battle and then discovers that he can merge it with his Susanoo - like form.
It could be huge and have unique markings. Maybe it would be colored in Naruto's signature orange to match his overall aesthetic.
Naruto would probably win. He has an indomitable will and has always found ways to overcome powerful foes. His experience and the unique powers he might gain with the Rinnegan could give him an edge over Madara.
Well, it really depends on how the fanfiction is written. But generally, the Rinnegan has extremely powerful abilities like the Six Paths powers. However, the Sharingan also has its own set of formidable skills such as predicting movements and copying jutsu. If Naruto with the Rinnegan can utilize the full potential of its powers like the ability to absorb and repel ninjutsu, he might have an edge. But Sasuke's Sharingan could potentially counter with its speed and precision in combat.
If Naruto got the Rinnegan, he might become more cautious. The power of the Rinnegan is so great that he would realize he needs to be more careful with it. He wouldn't be as brash as before.
Well, in many fanfictions, Naruto might first awaken Susanoo during a really intense battle. Maybe he's pushed to his limits while fighting a super - strong enemy, and his latent Uchiha blood (if that's part of the fanfic's premise) gets activated, leading to the awakening of Susanoo.
Their relationship would likely become more complex. With the Rinnegan involved, they might have to rely on each other more for understanding its powers. They could become more protective of each other, knowing the power and the danger it brings.
Well, at first it might cause some strain. His sister may feel overshadowed by Naruto's new power. But as time goes on, Naruto could use the Rinnegan to help his sister discover her own latent abilities. This would transform their relationship into one of mutual growth and support. He could use the powers of the Rinnegan to train her in unique ways, and she could in turn offer him emotional support, making their bond even deeper than before.
One way is through the power system. The Susanoo's power could be compared or combined with the powers in Highschool DxD. Naruto could bring his ninja way of thinking into the Highschool DxD world. For instance, his idea of never giving up could inspire the Highschool DxD characters. And in terms of the world - building, the fanfic could have Naruto's world of ninjas and the Highschool DxD's world of demons and angels co - exist. The Susanoo could be seen as a new and fearsome power in the Highschool DxD world.
One reason could be to explore a unique power combination. Naruto is known for his own powerful jutsu like the Rasengan, and combining it with the Susanoo, which is a very powerful Uchiha ability, makes for an exciting and fresh concept in the Naruto universe.