A role reversal in a Hunger Games fanfic would mean that the hunters become the hunted. The Capitol's technology and power would be in the hands of the districts' tributes. This could lead to a more chaotic and unpredictable Hunger Games. The Capitolites, unaccustomed to such brutality, would face a harsh reality. The tributes from the districts might show mercy or they might exact revenge. It would depend on how the author wants to write it. Also, the propaganda aspect of the Hunger Games would change. Instead of the Capitol using it to control the districts, the districts could use it to expose the Capitol's weakness.
If there was a role reversal in a Hunger Games fanfic, it could completely transform the narrative. For example, the Capitol citizens could be the ones forced into the arena while the tributes from the districts become the spectators. This would flip the power dynamic on its head. The Capitolites, used to luxury, would likely struggle with the harsh conditions of the arena, not having the survival skills the district tributes have. It would also change the themes of the story. Instead of a story about the districts' fight against the Capitol's oppression, it could become a story about the Capitolites realizing the cruelty of their own system when they are on the receiving end.