It could be a friendship of sorts. They are the same person after all, just from different time periods. They might share a deep understanding of each other's hopes and fears. For instance, the present Tsuna might be afraid of the future, but the future Tsuna can assure him that things will work out. And they could start to rely on each other in different ways, like the present Tsuna depending on the future one for advice, and the future Tsuna relying on the present one to make the right choices to ensure that future.
Perhaps they'll start as strangers to each other in a way, because the present Tsuna is still naive compared to his future self. But as they interact more, they could form a very unique bond. The future Tsuna might see his younger self's innocence and be determined to protect it, while the present Tsuna could look up to his future self as an ideal version of himself. They could end up working together to change or secure the future, with the future Tsuna sharing his wisdom and the present Tsuna providing fresh perspectives and enthusiasm.
They could develop a mentor - mentee relationship. The future Tsuna has more experience and knowledge, so he can guide the present Tsuna.
At first, it might be a bit tense. Natsu would be full of questions and his future self might be a bit reluctant to share everything. But as they start to talk, they could become more understanding of each other.
There are many possibilities. The future Tsuna could be there to warn the present Tsuna about making certain mistakes. He might reveal secrets that are crucial for Tsuna's development. For example, if there are some hidden powers or alliances that Tsuna needs to be aware of. Also, the encounter could be a turning point in Tsuna's character development, making him more confident and determined to face the future.
In a fanfiction, their relationship might start off rocky. Deadpool's crazy antics would likely put Peter off at first. But as they face challenges together, they could grow to respect each other. Deadpool might learn some sense of responsibility from Peter, and Peter could loosen up a bit thanks to Deadpool's carefree attitude. Eventually, they could become a great superhero duo, with Deadpool providing the comedic relief and Peter the moral compass.
They might develop a mentor - mentee relationship. If Craig is more experienced in the ways of the world outside the barbarian tribes, he could mentor Tweek on how to navigate different cultures and situations. Tweek, on the other hand, could be Craig's mentee in terms of physical strength and combat skills, which could come in handy for Craig when he gets into trouble.
A mentor - mentee relationship could develop. The Winchesters, with their years of experience in fighting all kinds of supernatural beings, could mentor Harry. They could give him tips on combat strategies, survival skills, etc. Harry, on the other hand, could learn a great deal from them and maybe even apply some of those skills in his own world.
They could develop a mentorship - like relationship. Lois, with her knowledge of the superhero world and her experience in dealing with complex personalities, could guide Conner. He, in turn, could offer her a fresh perspective on the world of superpowers.
They might develop a friendship. Harry could see Daphne's kindness beneath her Slytherin exterior at the Yule Ball, and Daphne could realize that Harry is not just the 'Chosen One' but also a normal boy with normal feelings. This new understanding could lead to a friendship where they support each other through the challenges at Hogwarts.
Deku could become more cautious. If his future self shows him some mistakes he made in the future, he would be more careful in his actions. For example, he might think twice before making a hasty decision in a battle. Also, he could gain a more mature perspective on life and hero - work, as he gets a glimpse of what lies ahead.
A mentor - mentee relationship could develop. Naruto has a lot of experience and skills, and Inari is young and eager to learn. Naruto could guide Inari in his ninja journey.
A possible relationship could be one of mutual respect. Sarada would respect Itachi for his skills and his past sacrifices. Itachi, on the other hand, might see Sarada as a symbol of hope for the future of the Uchiha clan. They could share stories about the Uchiha, with Itachi passing on the history and traditions that Sarada might not be fully aware of. This could strengthen their bond and lead to a deeper understanding between them. In the fanfiction, they might also team up to face a common enemy, relying on each other's strengths and abilities, which would further solidify their relationship.