It could be Voldemort. Since Snape is always in a complex relationship with Voldemort in the Harry Potter universe, even in a fanfiction with Rumplestiltskin and the mirror. Voldemort might see Snape's new connection with Rumplestiltskin's magic as a threat and try to stop him.
Another possibility is an ancient dark magic entity that is awakened by the combination of Snape's magic, the mirror's power, and Rumplestiltskin's presence. This entity could be interested in using their powers for its own evil purposes and would oppose Snape as he tries to prevent it from causing chaos in the wizarding world.
Maybe it's Rumplestiltskin himself. He might start off as an ally to Snape, but then his true nature of being a trickster could come out. He could demand more and more from Snape in exchange for his help, and eventually turn into the main force that Snape has to oppose in order to save himself and those he cares about.
One plot idea could be that in a magical mirror world, Severus Snape encounters Rumplestiltskin. Maybe Snape is seeking a way to reverse a dark spell he accidentally cast, and Rumplestiltskin offers to help in exchange for something precious to Snape, like his memories of Lily. Another could be that the mirror shows an alternate reality where Snape has the power of Rumplestiltskin's deal - making, and he uses it to change the fate of Hogwarts during the war.
James Potter is an obvious choice. His constant bullying of Snape made him a sort of antagonist in Snape's school years. His actions towards Snape were cruel and had a big impact on Snape's character development.
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.
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.
One possible antagonist could be a dark wizard from the past who somehow catches wind of Albus Severus' time - traveling and decides to use his knowledge of the future for his own evil purposes. Maybe a forgotten Death Eater who was thought to be dead but actually survived in hiding and now sees this as an opportunity to rise again.
Perhaps a group of rogue Hogwarts students who have fallen under the influence of dark magic. They want to use Harry for some dark ritual, and Snape becomes an unexpected shield for Harry. Snape has to deal with the internal problem of the school while also protecting Harry from these students.
It could be a dark wizard who is jealous of Snape's relationship with Hermione. This antagonist might try to break them apart by causing trouble in Hogwarts or spreading false rumors about them.
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.
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.
The girl, the king, and Rumplestiltskin are the main characters. The girl is the one who is put under pressure by the king. The king is the one who sets the impossible task of spinning straw into gold for the girl. And Rumplestiltskin is the key character who has the magical ability to turn straw into gold and uses this ability to bargain with the girl.