A good Curtis fanfic in Pokemon Black 2 would have engaging dialogue. The conversations between Curtis and other characters should feel natural and help to move the story forward. It should also have accurate descriptions of the Pokemon battles. If Curtis is in a battle, the moves, the Pokemon's behavior, and the overall flow of the fight should be well - written. And of course, a touch of creativity, like introducing a new Pokemon form or a special item that Curtis discovers, can make the fanfic stand out.
In a good Curtis fanfic for Pokemon Black 2, the author should really understand Curtis as a character. They should know his strengths and weaknesses and use that knowledge to create a believable story. The fanfic could also explore the Unova region in a new way. Maybe Curtis stumbles upon a hidden area with rare Pokemon and unique trainers. The story should also have some sort of conflict, whether it's an internal struggle for Curtis or an external threat like a new villainous team, to keep the reader interested.