Perhaps Orochimaru, if he somehow discovers Ino's time - traveling. He has always been interested in power and forbidden jutsu. He might try to use Ino's ability for his own gain, and if she resists, he would become an antagonist. He could also try to stop her from changing things that he wants to keep as they are for his own evil purposes.
One plot idea could be that Ino goes back in time to the founding of Konoha. She could use her knowledge of the future to prevent some of the early tragedies that befell the village. Another idea is that she goes back to her own childhood and tries to change certain events that led to insecurities in her later life. For example, she might try to befriend Sakura earlier and avoid the rivalry that developed between them.
She could meet her younger self. This would be an interesting encounter as she could offer advice and guidance to her younger version. Maybe she could help her younger self to be less afraid of her powers.
Well, they could also change their own relationships. Maybe Naruto wouldn't be so alone as a child, and Ino could become a stronger ninja earlier. They might train together in ways they didn't before, using the knowledge of future jutsu and strategies. They could also influence the relationships between different ninja clans, like improving the relations between the Uzumaki and the Yamanaka clans.
It could be the same antagonists as in the original Naruto series at first, like the Akatsuki. But if Naruto and Ino change things, new antagonists might emerge. Maybe some ninja who were once allies turn against them because they fear the changes Naruto and Ino are making to the ninja world order.
He could change the course of major events. For example, he might prevent the rise of a powerful villain. This would have a huge impact on the entire galaxy. Maybe he could save some characters who died in the original timeline.
One interesting aspect could be that Luke goes back to a time when the Sith were just emerging. He could try to root out the Sith threat before it became as powerful as it did. He might have to be very careful not to disrupt the timeline too much, but his Jedi wisdom could be a great asset in this time - traveling adventure.
In this fanfiction, Sora might change key events. Maybe he could prevent some of the tragedies that occurred in the original story. For example, he could save a character who was lost before.
One possible plot could be that Kagome goes back in time and tries to change a major event in the past. For example, she might try to prevent a big battle that had a lot of casualties. She could use her modern knowledge and skills to her advantage in the past, like using first - aid knowledge to save lives.
She might change important events in the wizarding world. For example, she could prevent certain dark magic incidents or save characters who died in the original timeline.