Well, you might start by exploring how the switch affects their relationships with other characters. Also, think about how they adapt to each other's skills and limitations. That could make for an interesting story arc.
You could have them try to find a way back to their original bodies while facing challenges that test their newfound abilities. Maybe there's a magical element involved in the switch that needs to be undone. It all depends on the tone and direction you want the fanfiction to take.
One way could be to focus on the confusion and adjustments the characters have to make. It can add a lot of drama and humor.