Often, other characters from the Naruto universe also make appearances. For example, the various ninja clans like the Uchiha and the Senju. Their loyalties and relationships with Indra and Asura can add depth to the story. Maybe there are also some original characters created by the fanfiction writer to interact with Indra and Asura and drive the plot forward in unique ways.
There could be new antagonists created specifically for this fanfic. Maybe some dark forces that were sealed away by Asura long ago are awakened. They would oppose Naruto because they see him as a threat to their resurrection or their evil plans.
Kaguya could be a great antagonist. Even though she was defeated in the main Naruto story, she could be resurrected or have some lingering influence. Her power - hungry nature and her desire to control everything could put her at odds with Indra and Asura, who are symbols of two different paths in the ninja world. She might try to manipulate them to further her own goals or eliminate them as threats to her reign.
His relationship with the Hidden Leaf Village as a whole would change. The village might see him as even more of a savior or a potential threat, depending on how they view Asura's legacy. Some might fear that Naruto's connection to Asura could bring about a great change, either for the better or for the worse. Naruto would have to deal with these new expectations and attitudes from his fellow villagers while still being true to himself.
One way to blend the worlds is through the concept of power. Asura Wrath has incredibly powerful and god - like beings. In a fanfic, you could have Naruto characters encounter these powerful entities. For example, Naruto himself could meet Asura and learn about a different kind of chakra or power source. This would not only introduce a new element to the Naruto world but also create an interesting dynamic as Naruto's ideology of never giving up might be put to the test against the seemingly invincible Asura.
In this fanfiction, Naruto becoming Asura could bring about a whole new level of power. It might change his relationships with his friends and foes alike. For example, his connection with Sasuke could take on a different dynamic as he becomes a more god - like figure. His abilities would likely be enhanced to an extreme degree, allowing him to face threats that were previously insurmountable.
Madara could also be a suitable antagonist. Imagine if he somehow came back to life and found out about Asura and Indra's reincarnation. He would surely try to use them for his own goals, or maybe try to prove that his way of thinking was right all along. He could also use his knowledge of the Sharingan and Rinnegan to create new challenges for Asura and Indra.
It could be Orochimaru. He's always been interested in power and with Naruto having the Asura power in this fanfic, Orochimaru might see it as a great opportunity to steal that power for himself and use it to create an even more powerful battle avatar.
There probably is. Fanfiction writers often come up with really creative crossovers. However, the quality can vary a great deal. Some might be really well - written with interesting plotlines that blend the Naruto and Soul Eater universes in a unique way, while others might not be as good. It's all about exploring different interpretations of the characters and their worlds.