One antagonist could be a corrupted version of one of Naruto's old friends. Maybe they were influenced by some dark force and now have powers that rival Naruto's. They would oppose Naruto not only because of the power of the kyoketsu shoge but also due to their twisted view of the ninja world. Also, there could be a group of interdimensional invaders who see Naruto's power and the kyoketsu shoge as a threat to their plans of conquering the ninja world. They would use their advanced technology and unique abilities to try and defeat Naruto.
The plot could involve Naruto being in a super - powerful form (godlike) and using the kyoketsu shoge in unique ways. Maybe he uses it to fight against powerful new enemies that are immune to his regular jutsu. He could also use it to protect his village in a more stealthy and unexpected manner compared to his usual flashy attacks.
Maybe his godlike state gives him superhuman reflexes, allowing him to handle the kyoketsu shoge knife with extreme precision. He can anticipate his opponents' moves better and use the unique features of the knife, like the chain, more effectively in combat.
He might face powerful ninja with unique abilities. These could be ninja who can manipulate elements in very dangerous ways, like creating huge firestorms or massive floods. Naruto would have to use the kyoketsu shoge knife cleverly to counter their attacks.
It could be the other Hidden Villages. Since Naruto is becoming so powerful under Danzo's training, they might see him as a threat and try to stop him. Maybe they would send their best ninjas to take him down.
There might be some ancient forces or entities that are awakened by Naruto and Sasuke's new 'godlike' status. These could be demons or spirits that were sealed long ago and now see Naruto and Sasuke as threats to their plans of domination. They would be very powerful and require Naruto and Sasuke to use all of their enhanced abilities to defeat.
Perhaps there are dark forces from another dimension. These forces might be attracted to Naruto's power and want to either steal it or destroy him to prevent him from using it against them. They could be powerful demons or sorcerers with their own motives.
It could be a new group of rogue shinobi who have been causing chaos in the ninja world while Naruto was away. They might have targeted Naruto's friends or family, which is why he's back for revenge.
Another possibility is that the main antagonists could be the leaders of the other ninja villages. They might view Naruto's godlike and rogue status as a danger to the balance of power among the villages. So, they could form an alliance against Naruto, leading to a lot of political intrigue and conflict in the story.
It could be a rogue ninja who is jealous of Naruto's godlike powers and wants to take them for himself. This ninja might target Naruko as a way to get to Naruto.
Well, in this fanfic, Naruto's use of the Speed Force could lead to some really interesting crossovers. Imagine him meeting characters from the DC Universe who are also associated with the Speed Force. He could learn new techniques from them or teach them some of his ninja skills. In terms of the Naruto world, his newfound speed could make him a more elusive target. He could use it to quickly gather intelligence across different villages without being detected, completely changing the power dynamics in the world of ninjas.