A plot twist could be that Slade has an ulterior motive for training Naruto. Maybe he plans to use Naruto as a weapon against Konoha in the future. He might be secretly feeding information about the village's defenses to an enemy while training Naruto. So, when Naruto discovers this, he has to find a way to stop Slade and also deal with the fact that he has been unknowingly part of a scheme against his own home.
Naruto's friends might be initially suspicious of the new skills and attitude he gets from Slade's training. For example, Sasuke could see Naruto's new combat style as too ruthless and this could create tension between them. Sakura might worry that Naruto is getting too influenced by a dangerous figure. However, if Naruto can show that he's still the same person at heart, they could gradually accept his new abilities.
Well, one possibility could be that Naruto, with his ninja skills and strong sense of justice, joins Slade for a common goal like fighting a greater evil that neither could face alone. Maybe they team up to take on a powerful, shadowy organization that threatens both their worlds.
Maybe a new character created specifically for this fanfic. A powerful sorcerer who wants to use Naruto's chakra and Slade's combat expertise for his own evil purposes. He could be manipulating events from the shadows, pitting Naruto and Slade against each other at first until they realize they are being played.
Kosuke might be a character created specifically for this fanfic. He could be someone with a unique background, like perhaps he was once a disciple of a great ninja but then went on his own path. And now he sees potential in Naruto and decides to train him. He could have abilities that are different from the main characters in the original Naruto series, which is why he has something new to offer to Naruto in terms of training.
In this fanfic, Lilith could be an original character created by the author. She might be a powerful and mysterious figure with special abilities that make her suitable to train Naruto.
She could be someone with a special bloodline or ability. Maybe she has a unique form of chakra that gives her certain powers. In the context of training Naruto, her skills and knowledge are what make her important. She could be a mysterious figure who has been observing Naruto for a while and decides to take him under her wing to help him reach his full potential. For instance, she might have the ability to see the latent potential in others and wants to shape Naruto into a great ninja not just for his own sake, but also for the greater good of the ninja world.
It could lead to Naruto becoming even stronger and more skilled than in the original story. Jiraiya's training methods might unlock new powers for him.
It could be really exciting. Guts' brutal and unyielding training methods might make Naruto even stronger. Naruto could learn to face his enemies with more tenacity from Guts. For example, Guts' way of fighting in close combat could be incorporated into Naruto's combat style, adding a new dimension to his abilities.