In such a fanfic, perhaps Naruto is betrayed by his village and left in the forest. Maybe there are enemies lurking in the forest that he has to face alone. It could also involve Naruto's struggle to survive and his journey to find out why he was abandoned like this.
Perhaps some of the other ninja villages see Naruto as a potential danger in the future, so they manage to infiltrate Konoha and make it seem like Naruto has committed a great crime, resulting in him being left in the forest. In this case, the main antagonist would be the ninja from the other villages.
One possible plot could be that Naruto discovers a hidden conspiracy within the Forest of Death that led to him being left there. Maybe some rogue ninja or a hidden enemy within the village set him up.
I'm not sure specifically as there could be many different 'naruto is left to die' fanfics with various plots. But generally, it might involve Naruto being in a situation where his comrades abandon him or he is placed in a deadly situation by some malicious force, and then he has to struggle to survive on his own.
Danzo could be the main villain. He is known for his underhanded tactics and his dislike for Naruto. He might see Naruto as a threat to his power and plan to get rid of him by leaving him in the Forest of Death. Danzo could have manipulated some other characters to make it seem like it was a legitimate mission gone wrong.
Obviously, Naruto would be one of the main characters. Depending on the specific plot, Sasuke could also be involved. He might be one of the ones who either helps Naruto or is part of the reason Naruto is left to die. Sakura could also play a role, maybe she has a change of heart towards Naruto in the fanfic.
In some 'naruto left to die fanfic', the plot may revolve around Naruto being betrayed by those he thought were friends. This betrayal could lead to him being stranded in a place where he's left to fend for himself. He may then have to overcome his emotional turmoil and find a way to get revenge or prove his worth.
Kakashi could also be involved. Maybe he's put in a difficult position where he can't save Naruto for some reason, like being under a powerful genjutsu or being blackmailed. This would create an interesting conflict as he's Naruto's teacher and mentor.
I haven't read this 'birth of the forest naruto fanfic' specifically. But generally, in a Naruto fanfic, it might explore new storylines within the Naruto universe. Maybe it focuses on the birth of a special forest that has some sort of connection to the ninja world, like it could be a place full of rare chakra - rich plants or a hidden area where new forms of jutsu are discovered.
The two main characters are Naruto and Rias. Naruto, with his ninja skills and his journey of growth in his original story. Rias, whose actions towards Naruto are the central focus of this fanfiction. There could be side characters like Naruto's friends from his world who might try to find out what happened to him, or characters related to Rias who could either support or oppose her decision to leave Naruto to die.