It would really make sense if the Scarecrow was Batman’s archenemy. He’s a deep thematic foil who directly confronts fear, which is the core of Batman’s crusade. Also, he forces Batman to confront his deepest psychological traumas, and when he unleashes his fear toxins, the threat expands from an isolated crime to the whole city gripped by mass hysteria and phobias, which requires Batman to master his own mind to save Gotham.
Batman and the Scarecrow are two sides of the same coin, as they both use fear as their weapons. Except Batman uses it for justice, while the Scarecrow uses it for evil. The Scarecrow should definitely be Batman’s archenemy.