He might start by observing. Aang is a curious person, so he'd likely watch how people live, how they interact with technology. He may also try to find other benders to learn about any new developments in bending. And he'd probably use his kind heart to make friends easily in this new future environment.
In this fanfiction, Harry might see a world where magic has evolved in ways he couldn't imagine. Maybe there are new spells and potions that are far more advanced. He could also encounter the future versions of his friends and enemies. For example, he might find out that Ron has become a very important figure in the wizarding world, or that Voldemort's influence still lingers in some unexpected ways.
Maybe Hamilton would see a future where the ideals he fought for have been fully realized. He might be amazed at the technological advancements and how they have been used to uphold equality and freedom.
Well, it could be about Sakura finding herself in a future world full of advanced technology. Maybe she has to adapt to new ways of life and use her skills from her own time to solve problems there.
In this fanfiction, Naruto might see a world where his friends' dreams have fully come true. For example, Sasuke could have achieved true peace in the ninja world in his own way. And Konoha might have advanced in technology, with new kinds of jutsus being developed that combine the old and the new.
The older characters such as Kakashi would be cautious. They would recognize Naruto but also be worried about the implications of someone from the future being in the present. They would want to know if his presence could change the future in a negative way. For example, they might fear that his actions could lead to the earlier awakening of a powerful enemy. As for the villains, they might see future Naruto as a threat and try to eliminate him as soon as possible. Orochimaru, for instance, would be very interested in Naruto's future powers and might try to capture him to study. And the new generation of ninja in the past would look up to future Naruto, seeing him as a symbol of what they could become.