After some controversy, Nintendo has told USGamer that the amiibo would be used to unlock a Fusion Mode AND a Hard Mode, and only one of these modes will be completely unlocked behind amiibo. Upon completing the game, players will have access to Hard Mode; using amiibo will allow both modes to be unlocked immediately.
Hard Mode will become available once you have completed the main game, but Fusion Mode is locked behind an Amiibo.
Along with the Fusion mode being locked behind the amiibo, galleries will also be locked behind them.
– Ali –