Luna Lovegood could be an interesting choice. Her unique and somewhat eccentric personality could complement Snape's brooding nature. She might see through his tough exterior and find the good in him, and their relationship could be based on mutual understanding of the 'outsider' status they both have in different ways.
Hermione Granger is another possibility. Given her intelligence and determination, she could have a relationship with Snape based on their shared love for knowledge. She might challenge him intellectually and also help him to grow as a person, despite their age difference.
The other Death Eaters could be antagonists. They might not trust Snape completely and try to undermine him. For example, Bellatrix Lestrange could suspect Snape's loyalty to Voldemort and try to expose him while he's alone, which would put him in a very dangerous situation.
Luna Lovegood could be an interesting love interest. She has an open - minded and eccentric nature. Her ability to see the good in people, even those like Snape who seem cold on the surface, could draw them together. She might be able to make Snape appreciate the beauty in the world in a way he never has before.
Well, Harry Potter would likely still be a main character. His relationship with Snape could be even more complex in this fanfiction. Since Snape is a demigod, Harry might be both in awe and a bit scared of him at times.
It could be a completely original character created by the fanfiction writer. This gives the writer a lot of creative freedom to design a character that can have a unique relationship with Snape.
A powerful dark entity or an ancient evil awakened in the wizarding world could also be the antagonist. This entity might sense Snape's importance in the fight against the dark side and target him. It could use dark magic to incapacitate Snape, and Moody, being one of the few who can sense the danger, comes to his rescue. The entity could be something that even Voldemort was afraid of or something that has been long forgotten but now resurfaces to cause chaos.
Naruto would likely be more powerful in a physical combat sense. His training as a ninja has made him extremely strong, fast, and able to take a lot of damage. He also has the Nine - Tails' chakra at his disposal. But Severus Snape is no slouch. His knowledge of magic is extensive, and he could potentially use magic to outwit Naruto. For example, he could use spells to create illusions or disable Naruto's movements. However, Naruto has shown great adaptability in combat, and his willpower is unrivaled. So overall, Naruto's physical and spiritual power might give him the edge.
Bellatrix Lestrange could also be a character who interacts with Snape. If Snape goes back far enough, he might try to influence Bellatrix's path and keep her from becoming such a devoted Death Eater. This could have a major impact on Voldemort's power and the overall balance of the wizarding world.
Magic could play a role. There could be a magical spell or potion that was involved in Hermione's conception that links her to Snape. For example, a potion that alters memories and relationships. This would make for a very interesting and complex backstory in the fanfiction. Snape might not even be aware that he is Hermione's father at first, and when he discovers it, his whole view of her would change. He would have to deal with his feelings of protectiveness and his past actions towards her in Hogwarts.
In fanfiction, Severus Snape's boggart could be the lifeless body of Lily Potter. Since he loved her deeply and her death was a great tragedy for him. It would represent his ultimate fear of losing her all over again, even in the form of a boggart.
One possible plot could be that Snape discovers an ancient spell book related to Merlin. This spell book might contain knowledge that could help him in his mission to protect Harry Potter. For example, he could find a spell that would allow him to better conceal his true intentions from Voldemort. He might also learn about a long - forgotten potion that Merlin created which has the power to heal wounds that are otherwise considered fatal. Snape could then use this knowledge to save the life of a Hogwarts student during a battle.