An ancient and powerful entity that has awakened from a long slumber. It could be some sort of dark psionic being that wants to consume all life in the galaxy, starting with the Protoss and the OC's army.
Well, perhaps the OC could lead the Protoss army to victory in a great intergalactic battle. They might discover new technologies together and expand their territory across the galaxy.
Another great Protoss OC fanfiction idea could be set during a time of great crisis for the Protoss race. A Protoss OC, perhaps a young zealot - in - training, is thrust into a leadership role when the more experienced Protoss are incapacitated. The OC has to draw on all of their training and instincts to lead their brethren through the crisis, all while dealing with their own self - doubts and insecurities.
Some StarCraft - themed forums might also be a source for Protoss OC fanfiction. These forums often have sections dedicated to fan - created content. Members of the community might share their own stories or recommend ones they've read. For example, the official StarCraft forums or some of the more popular fan - run forums.
Perhaps another reborn character could be the main antagonist. Say, an evil sorcerer from the past who was also reborn in the Inuyasha world at the same time as the OC. This sorcerer has his own agenda of ruling the world and sees the OC as a threat to his plans, especially if the OC is getting in the way of his attempts to gather the Shikon Jewel fragments.
Perhaps there is a dark force that is awakened by the oc's reincarnation. This force might have been dormant for a long time, and the oc's new life energy or something about their reincarnation stirs it. It could be a force that wants to destroy the balance of the pjs world, and the oc is the only one who can stop it because of their unique perspective from their past life.
Maybe there is a new character created specifically for this fanfiction as the main antagonist. This new character could have a connection to the oc's past life or the reason for their rebirth. They might be trying to prevent the oc from fulfilling a certain destiny or unlocking their full potential.
It could be another powerful ninja who sees the student as a threat to their own power or plans. Maybe someone who doesn't want Madara to pass on his knowledge to an outsider or a young upstart.
Another possibility could be an original antagonist created specifically for the fanfiction. For example, a powerful sorcerer from a hidden dimension who sees the OC's rebirth as a disruption to the natural order of the MCU universe. This sorcerer could try to eliminate the OC and use their power for their own dark purposes. They might be able to manipulate magic in ways that even Doctor Strange hasn't seen before, making them a formidable foe.
It could be a new character created specifically for the fanfiction. Maybe a powerful pirate who has a vendetta against the Straw Hat crew because of the oc's presence. Since the oc is reborn into this world, they might bring some sort of change that angers this new antagonist.
It could be a dark wizard who has a vendetta against Erza and sees her son as a way to get revenge. This antagonist might try to corrupt the OC son or use him as a pawn in his evil plans.