Maybe it's an alien - like being from another dimension that is reborn in the Starcraft system. This being might have some unique abilities from its own dimension that it combines with the Starcraft system powers. It could bring a fresh perspective to the Starcraft universe as it tries to find its place and purpose in this new world.
It could be a human from our world who was a huge Starcraft fan. They die in some accident or strange event and are then reborn in the Starcraft universe as the oc. Their prior knowledge of the game could give them an edge in understanding the Starcraft system.
The plot might involve the oc having to adapt to the new and often dangerous environment of the Starcraft world. They could encounter different races from the game, such as the Terrans, Zerg, and Protoss. The oc may form alliances or face off against these races while using their newfound Starcraft system powers to survive and thrive. For example, if they are reborn as a Terran - like character, they might have to deal with political intrigue within the Terran factions while also fending off Zerg attacks with their strategic building and unit control skills.
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.
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.
The main character, the reborn OC, is a must. Then there could be a local food critic who has the power to make or break the bakery's reputation. This character could be very strict and demanding. There might also be a fellow baker from a neighboring town who is jealous of the OC's success and tries to sabotage the bakery. And let's not forget the OC's pet, if they have one. It could be a cat that lounges around the bakery, adding a cozy and homey feel to the place.
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.
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.
Superman could also be a main character. He might be the one to first discover the oc after their rebirth. Superman's moral compass would lead him to either protect the oc or keep an eye on them depending on how the oc behaves. And his super strength would be a factor in any interactions with the oc.
Lex Luthor could be a main antagonist. If the oc is trying to promote a new form of clean energy or something positive in Metropolis, Lex would see them as a threat to his own power and business interests. He'd use his intelligence and resources to try and stop the oc.
It could be an ancient evil entity that has been sealed away for centuries. The OC's rebirth somehow disturbs the seal, and this entity awakens. It views the Fairy Tail guild as a threat because of their positive influence in the world. So, it starts to target the guild, and especially the OC, as they are a new and unknown factor in the fight against it.
It could be an old - fashioned vampire who believes in the strict hierarchy and doesn't want any new elements, like the reborn oc, to disrupt the status quo. They might see the oc as a threat to their power and influence in the vampire community.