In a fanfiction, if Draco is younger, it could also affect their relationship with their families. Harry has his own complex family issues, and Draco being younger might mean his family is even more protective or controlling. This could lead to Draco rebelling in different ways than he does in the original story. Maybe he sees Harry as a symbol of freedom or a different way of living, which could drive an interesting plotline in the fanfic.
One implication could be in terms of power dynamics. Since Draco is younger, he might be more naive or less experienced in the ways of magic compared to Harry. This could lead to situations where Harry has to protect Draco, or Draco looks up to Harry in some ways, even if they are on opposite sides in the original story. For example, in a fanfic where they are forced to work together, Draco's younger age might make him more impressionable and open to changing his views because of Harry's influence.