One possible plot for this fanfiction could be that an ancient power in the Naruto universe is actually related to the Ultraman power source. Naruto, being the curious and adventurous ninja he is, investigates this connection. As he delves deeper, he accidentally merges with the Ultraman power. Now, he not only has to protect his village from the likes of Orochimaru and Akatsuki but also has to face off against the giant monsters that Ultraman typically battles. He would have to adapt his fighting style and also find a way to keep his new identity a secret from his friends and villagers who might not understand his new form and powers.
One interesting crossover could be Naruto obtaining Ultraman's powers. Imagine him using the Spiral Zankai Beam instead of the Rasengan. It would be a powerful and unique addition to his fighting style.
Often, they start off as strangers. Ultraman characters might be confused by the ninja world's rules and customs, while Naruto characters are in awe of Ultraman's powers. For example, Naruto might be excited to learn about Ultraman's beam attacks and try to incorporate the concept into his own jutsu training.
The plot could also involve Naruto trying to convince the Raikage to change some of the Cloud Village's more isolationist policies. As Naruto has a vision of unity among all the ninja villages, he sees the Raikage as a key figure to help him achieve this. The Raikage, initially resistant, might gradually come around to Naruto's way of thinking as they spend more time together.
Well, one possibility could be that Naruto somehow gains the ability to perform bankai. He might use it in a battle against a powerful new enemy. For example, he could combine his own ninja skills with the unique powers of bankai to create a whole new combat style that surprises his foes.
Perhaps in this fanfiction, Naruto discovers that he has some unique weapon - like abilities that are different from his normal jutsu. It could lead to a whole new adventure where he has to learn to control this 'weapon' aspect of himself while still being true to his friends and the village.
Well, one possible plot could be that Naruto discovers a new form of chakra that is related to digital energy. He then uses this to hack into systems to prevent a global digital meltdown that some evil hackers are planning. He teams up with his friends, who also have unique digital abilities, and they fight against the bad guys in the virtual world.
One possible plot could be that Naruto is somehow reincarnated as Fenrir. He might retain some of his ninja abilities but also gain the powers of Fenrir like super strength and the ability to change forms. He could then use these combined powers to protect the Hidden Leaf Village from a new and powerful enemy that normal ninja techniques can't defeat.
Perhaps Naruto is injured while on a solo mission to infiltrate an enemy stronghold. He gets caught and tortured, which leaves him severely injured. Sakura and the others then launch a rescue mission, and during the journey back, they face many challenges in keeping Naruto alive and treating his wounds.
The plot could be centered around a mystery. Yuriko discovers a strange artifact and Naruto, being the curious ninja he is, joins her to figure out what it is. Along the way, they face opponents who also want the artifact for their own malicious purposes.
The plot could involve Naruto traveling to the world where the Undertaker exists. There, he could learn some dark arts or special fighting techniques from the Undertaker. He might also form an unlikely alliance with the Undertaker to fight a common enemy that threatens both the ninja world and the Undertaker's domain. This would lead to Naruto having to adapt to a completely different set of rules and powers.