There could be demons with special knowledge. Naruto might meet a demon who knows about ancient ninja secrets or prophecies. This demon could either help or hinder Naruto depending on its motives. He could also run into demons that are in exile from the demon world, and they might be more willing to cooperate with Naruto as they see an opportunity to return to their world through the portal he opened.
One potential consequence could be the release of powerful demons into the Naruto world. This would disrupt the existing balance of power. For example, villages that were previously in a state of relative peace might have to unite against the new threat. The ninja way of life could change as they adapt to fighting these otherworldly demons. Also, Naruto himself might face internal struggles as he realizes the full extent of what he has done by opening the portal.
It could be a rogue ninja who has made a pact with the demons in the demon world. This ninja has turned against the ninja world and wants to use the power of the demon world to rule over all. He would constantly oppose Naruto's attempts to restore peace.
It could be an old rival of Naruto's who has fallen to the dark side and now uses a masked form to hide their identity. Maybe someone like Sasuke if he had been further corrupted by Orochimaru's teachings. Another possibility is that it's a new character created specifically for the fanfiction. This new character could be a rogue ninja from a far - off land who has a grudge against the Hidden Leaf. Or perhaps it's a resurrected villain from the past, like Madara Uchiha, who has been transformed into a masked demon through some dark jutsu.
He could meet female versions of his existing friends like a female Sasuke or Sakura. They might have different personalities while still retaining some of the traits of their male counterparts. For example, a female Sasuke could be just as strong - willed but with a more feminine touch in her fighting style.
One possible aspect of a Naruto Demon Negima fanfic could be the combination of different power systems. In Naruto, there are chakra-based abilities like ninjutsu and genjutsu. In Negima, there are magic spells. In a fanfic, characters might learn to combine these powers, for example, a Naruto character using a Negima - style magic spell enhanced with chakra. This could lead to really interesting battles and new storylines as they face new and unexpected enemies.
There could be a group of kids on the beach who are inspired by Naruto. They might approach him and ask for autographs or to show them some ninja moves. Naruto would be happy to oblige and in the process, he could discover a new talent among the kids that could be important for the future of the ninja world.
It could be an ancient demon lord who wants to enslave both the demon prince and Naruto. This demon lord might see the unique bond between them as a threat to his plans of domination.
Obviously, the demon overlord could be the main antagonist. He might have his own motives like ruling the ninja world or obtaining some powerful jutsu that Naruto and his friends are trying to protect.
Naruto is sure to be a central figure. Then there could be a new character, a half - demon who acts as a bridge between the ninja world and the demon army. Hinata might also be involved, perhaps using her Byakugan to discover secrets about the demon army. And of course, Kakashi could be there to guide Naruto and the others in dealing with this new threat.
One possibility is that they could have a story full of adventure. As demons, they might face challenges from the ninja world that fears them. But their love for each other gives them the strength to stand together. They could go on quests to prove that not all demons are evil, and in the process, their relationship deepens.