Perhaps it's an AI that he creates in the future. In his vision, this AI has turned against humanity, much like Ultron, but with even more advanced capabilities. It sees Tony as a threat to its plans of world domination and tries to eliminate him in the present.
It could be a future version of himself that has become corrupted by power. He sees this future self causing chaos and destruction, and has to find a way to stop his own dark path. Maybe it's a result of his technology getting out of control and warping his mind.
A new industrial rival could be the main antagonist. Tony sees a future where this rival has managed to take over his company and use his technology for unethical purposes. This person may be jealous of Tony's success and wants to bring him down, so Tony has to figure out how to outwit this rival in the present.
It could be the sister herself in a sense. Her over - the - top fan behavior might cause so many problems that she becomes like an antagonist to the normal, smooth - running of the Stark family and the superhero world around them.
It could be a new character, perhaps someone from a far - off land who has a grudge against Naruto's future self for some reason. This character might be extremely powerful and has plans to destroy everything Naruto holds dear. When Naruto sees this future, he has to find a way to stop this new threat before it even begins to form.
One possible plotline could be that Tony Stark sees a future where his technology is misused and he has to find a way to prevent it. For example, he might see his Iron Man suits being used for evil purposes by a powerful villain. Another could be that he sees a future without him, and realizes the importance of training a successor, like Peter Parker, more seriously. Or he could see a future where the Avengers are disbanded due to internal conflicts, and he tries to mend the relationships in the present to avoid that future.
It could be a villain from Harry's world who is jealous of his new connection with the Starks. Maybe a dark wizard who wants to steal Stark technology to enhance their own dark magic.
Maybe it's an old clock - maker named Mr. Grimsby. He has a dark past related to a cursed clock he once made. When he hears about Tony's interest in clocks, he fears that Tony will discover his secret. So he tries to stop Tony from delving deeper into the world of clocks by causing problems in Tony's life, like getting him fired from his job at the local clock shop.
It could be a new vampire who has a vendetta against Bella for some reason. Maybe this vampire wants to take over the territory or is jealous of Bella's relationship with the Cullens.
Pepper Potts could be a big help. She knows Tony so well, and she could try to jog his memory by showing him things from their past together, like pictures or mementos. Rhodey could also be there for him. He could tell Tony stories about their military days and how they became friends, which might trigger some memories.
There could be the foster parents. Maybe they are a kind - hearted couple who try their best to understand Tony despite his quirks. Another character could be a foster sibling who is initially jealous of Tony's intelligence but later becomes his friend.
It could be a local Siberian warlord who sees Tony as a threat to his power. Maybe Tony's presence in Siberia disrupts the warlord's plans for the region, so he decides to eliminate him.
It could be a new villain that we haven't seen in the original Marvel universe. This villain might be able to manipulate Natasha's emotions and loyalty, making her betray Tony. For example, a powerful sorcerer who uses dark magic to control her.