He could meet his future self. It would be an interesting encounter as he could learn from his future experiences and wisdom. Or he might meet people who have studied his past adventures and see himself through their eyes.
Possibly they are random villagers who were caught in the crossfire of a conflict between different bending factions. Aang, being the Avatar and a symbol of peace, feels responsible for burying them and ensuring their spirits are at peace.
Sokka could be one of the main characters. He might be the one helping Aang with his schoolwork, especially in subjects like history or math, using his own knowledge and quick thinking.
Sokka would also be a common character. His strategic mind could be very useful in a different time period. He might be the one who figures out how to deal with the new situations they encounter while in the past. For instance, if they need to outwit an enemy in a historical battle, Sokka would be the one to come up with a plan.
Aang could be friends with Neville. Neville is often underestimated but has a lot of courage, just like Aang. They could bond over their shared experiences of being different. Also, Luna Lovegood might be interested in Aang's unique abilities and his different world view, so she could be a potential friend too.
Obviously, aang's parents would be the main characters. But there could also be other important characters like their friends in the tribe. Maybe there's an old wise man who gives them advice on raising Aang. Or a rival from another tribe who later becomes an ally.
Aang is surely involved in his own love story. In the Avatar world, there are several characters that could be connected to his love life. For example, Katara, with whom he shares a special bond. Their relationship develops over time, from friendship to something more. There may also be other characters who play minor roles in influencing Aang's view on love and relationships.
Katara could be one of the main characters. She might play a role in helping Aang through the process of opening his chakras, either with her healing abilities or just by being there to support him emotionally.
Aang might go into a deep state of depression. He has always been an emotional character, and having his heart broken by Katara, someone he loves deeply, could send him into a downward spiral. He might isolate himself from his friends and stop practicing his bending for a while.
In some fanfiction, Aang's tattoos might be depicted as glowing with a special power when he reaches a new level of understanding or connection to the Avatar state.