Pandemic on normal difficulty is solvable by an experienced four-player team in under an hour with high reliability. This is a feature for teaching purposes and a limitation for experienced players seeking challenge. The Heroic difficulty with full epidemic cards changes the math substantially.

The On the Brink expansion is the best way to add challenge for experienced groups. The virulent strain challenge adds a super-disease that requires additional attention. The bioterrorist challenge adds a hidden player working against the team. The mutation challenge adds a fifth disease.

For groups that have played Pandemic to mastery and want more challenge without switching games: On the Brink virulent strain is the correct addition. It maintains the accessibility while raising the ceiling significantly.