I've always hated the Borg Queen, as I think it completely undercuts the key property of The Collective, the collectiveness. So in my head canon, the queen is a result of adapting to Hugh's return to the collective. When Hugh is returned, we know The Collective (or at least part of it) turns into a motley crew of individuals desperate for a leader. So in my head, the uninfected part of The Collective creates the Borg Queen to serve as this leader, thus adapting to "individuality virus" that Picard inadvertently unleashed.