If Bilbo travels back to a time when his family was alive, his relationship with them would be transformed. He would have a chance to appreciate them more, and perhaps he could use his future knowledge to improve their lives. He might also struggle with not being able to fully share his experiences from his original timeline, which could add an element of melancholy to their interactions.
If Bilbo goes back in time to when he first met the dwarves, his relationship with them would be quite different. He would already know them well, and they might be more suspicious of his prior knowledge. But he could also use this to his advantage and form even stronger bonds with them more quickly, as he would understand their personalities better from the start.
One plot idea could be that Bilbo goes back in time to the founding of the Shire. He could use his knowledge from his own time to help shape the early laws and traditions in a more positive way. For example, he might introduce better farming techniques he learned from his travels, which could lead to the Shire being even more prosperous in the long run.
If Merlin travels back in time, his relationship with Arthur would be very different. In the past, Arthur might not trust him at all at first. Merlin would have to prove himself all over again. With Gwen, he might be more aware of her future importance and try to protect her in a different way. And his relationship with the other knights could be completely new, as they might see him as a stranger with strange powers.
If Natsu goes back in time, his relationships would be very different. With his friends, he might have a new level of understanding. For example, he could see how they grew up and the challenges they faced in the past. He might be more protective of them, especially if he knows what's going to happen in the future. His relationship with Lucy could also change. He could see her in a new light, perhaps realizing things about her that he didn't before when he first met her in the present.
If Ianto travels back in time, his relationships with the Torchwood team in his original time would be put on hold. They would be left wondering where he went. In the past, he might form new relationships. For example, he could become friends with an ancestor of one of his present - day colleagues. This would create a strange connection between the past and the present for the team when he eventually returns.
His relationship with Kakashi would also change. Naruto could use his Rinnegan - enhanced knowledge to help Kakashi deal with his past traumas earlier. He might also show Kakashi new jutsu or ways of combat that he learned in the future. This would make Kakashi see Naruto in a whole new light and their bond could become more of a teacher - student relationship in reverse.
With his Slytherin peers, Draco's relationship would shift. He might encourage them to be more inclusive and less focused on pure - blood supremacy. This could make him an outcast among some of the more traditional Slytherins, but also gain him new allies who are open to change.
His relationship with his father Poseidon would be different. He might see a side of Poseidon that he never knew before, like when Poseidon was younger and less experienced. This could change how Percy views his father and their connection.
His relationship with his father, Ned Stark, would be different. He could finally have a real father - son relationship instead of the hidden one. He might be able to learn from Ned and become an even better leader in the North.
His relationship with Katara would be very different. He might be able to approach her in a more confident way, knowing what they've been through in the future. He could also avoid some of the misunderstandings that happened between them in the original timeline.
Regarding his relationships with his friends and comrades, like Samwell Tarly, things would be altered. He might not meet Sam in the same way as before. And his relationship with Daenerys Targaryen could be completely different. Instead of meeting her during the events of the original story, he could meet her when she was much younger, and this could lead to a very different kind of relationship, maybe one that's more of a mentorship or a completely different romantic connection.