There's also the possibility that an ancient dark magic entity, which has some connection to the Grim, is the main antagonist. It tries to manipulate Harry into using the Grim's power in a way that will free it from its ancient prison or some such situation, and Harry has to resist this while still maintaining control over the Grim.
One possible plot could be that Harry discovers some ancient magic that gives him the power to control the Black family Grim. He might use this power to uncover secrets within the Black family that were long hidden. Maybe he uses it to protect the family from dark forces that are after their power or knowledge related to the Grim.
It could be a dark wizard who also has an ability related to metal. He wants to take Harry's power for himself and rule the wizarding world. For instance, he might be able to corrupt the metal in Hogwarts and turn it against the students.
It could be a dark wizard who also wants to control the zombies but for evil purposes. Maybe a long - lost relative of Voldemort who has emerged from the shadows and sees the zombies as a means to finally take over the wizarding world. He might try to steal Harry's method of controlling them or create more powerful zombies that are immune to Harry's control.
There could be a plot point about Harry's relationship with the Black family's house - elf or other magical creatures associated with the family. He might have to deal with their loyalty issues. And perhaps there are other external forces that try to use Harry's control over the Black family for their own purposes. This could lead to a series of conflicts and challenges for Harry as he tries to keep the Black family under his control while also protecting them from outside threats.
There could be a powerful shinobi from within Konoha who is jealous of Naruto's unique ability to control snow. This shinobi might secretly work against Naruto, trying to undermine his progress and steal the power for himself.
It could be an ancient evil that has been lurking in the shadows for ages. Maybe a dark spirit that was sealed away long ago and is now breaking free, and Naruto has to control the darkness within himself to fight it.
It could be a rogue Huntsman who wants to use Ruby's power to control Grimm for his own evil purposes. He might believe that with the Grimm under his control, he can rule over Remnant. He would then be constantly trying to kidnap Ruby or figure out the secret behind her power, and would oppose team RWBY at every turn.
It could be another character who also has some time - related powers but uses them for evil. This character might try to steal Percy's time - controlling ability or use their own powers to disrupt the timeline for their own gain.
It could be Orochimaru. He's always been interested in new and powerful jutsus. He might see Naruto's shadow - controlling ability as something he wants to steal or use for his own evil purposes.
A powerful dark spirit might be the main antagonist. This spirit could sense Naruto's new ability to control darkness and see him as a threat. So it starts causing chaos in the ninja world, forcing Naruto to use his dark - controlling powers to stop it. The spirit could have the ability to possess other ninjas, making them do its bidding.