In fanfiction, when Sabo and Luffy meet again, they might have an emotional reunion. They could share stories of their adventures since they last saw each other. Sabo might show off his new skills or powers, and Luffy would be excited to see his brother again. They could also plan new adventures together, vowing to face any challenges as a team.
Stories have been around for a very long time. It's hard to pinpoint an exact moment when they were first introduced as they likely emerged gradually throughout human history.