Maybe it's a powerful shinobi from another village who is testing the defenses of Konoha. Kotetsu and Izumo happen to be in the wrong place at the wrong time and become the main targets of this shinobi's schemes. He uses his powerful jutsu to create the nightmare situations for them.
There could be an internal antagonist within Konoha itself. A shinobi who is jealous of Kotetsu and Izumo's friendship and their position as gate guards. This shinobi uses dark jutsu to create the nightmares for them, hoping to drive them apart and take their positions.
It could be a rogue ninja who has a vendetta against Konoha. This ninja might target Kotetsu and Izumo specifically because they are guards of the village gates. They see them as easy targets to start their revenge plan against Konoha.
Well, it could be about Kotetsu and Izumo facing a really tough mission that turns into a nightmare. Maybe they get trapped in a genjutsu created by a powerful enemy. In this genjutsu, they relive their worst memories, like failing important missions in the past. And they have to find a way to break free together, relying on their friendship and the skills they've developed over time.
Well, Naruto would likely be one of them. He has a big heart and would surely want to help his fellow villagers. Sakura could also be involved as she is a skilled medical ninja. She could use her medical knowledge to diagnose and treat their illness.
A common theme is friendship. Kotetsu and Izumo are often depicted as having a strong bond in fanfics. Another theme is their loyalty to Konoha. Their role as guards and their commitment to the village are frequently explored.
One possible plot could be that while they are sick, their comrades in Konoha come together to take care of them. Maybe Naruto brings them some of his favorite ramen, thinking it might cheer them up and help them get better.
One popular 'naruto kotetsu and izumo fanfic' is 'Kotetsu and Izumo's Unseen Missions'. It delves into their daily tasks in Konoha that are often overlooked in the main series. Another great one could be 'The Bond of Kotetsu and Izumo' which focuses on their friendship and how it has developed over the years.
Another jutsu Naruto could learn is a defensive one. Given that Kotetsu and Izumo have to protect themselves and others, they might teach Naruto a jutsu that creates a stronger shield or barrier. This would enhance Naruto's survivability in battles, especially against powerful opponents. It could be a jutsu that combines chakra control and some form of elemental manipulation, which Naruto can then adapt to his own style with his large chakra reserves.
It could be a fusion of Orochimaru and Freddy Krueger. Orochimaru's love for experimenting and getting into people's minds combined with Freddy's nightmare - inducing abilities would make a terrifying antagonist. He could be using the nightmare world to steal the chakra of ninjas while they sleep, and Naruto has to stop him.
The Organization in the Claymore world could be the main antagonist. They already have a lot of secrets and do some morally ambiguous things. If Naruto shows up and starts changing the status quo, like teaching Claymore warriors to be more independent, the Organization might see him as a threat and try to eliminate him.
It could be a rogue ninja who was once a student of Orochimaru and has now developed his own dangerous jutsu. He wants to take over the ninja world and believes the old ways are holding everyone back.
Orochimaru could be a great antagonist. He might want to use Shenron's power for his own dark experiments. He could try to steal the dragon balls and manipulate the wishes to gain immortality or create an army of powerful, immortal ninja.