New unlockable game modes.

I think there should be something to get for doing things like beating the game and completing advancements.

After you beat the game for the first time you unlock hardcore mode. (side note hardcore needs something new)

After beating hardcore mode you unlock knight mode. (same as hardcore but once the first day is over it is night forever)

After conquering that, you unlock 3 new modes, Underground, dessert, Nether.

After beating each one of those, you get 3 different capes, the stone cape, the cactus cape, and the fire cape.

After getting all (non hidden) advancements, you unlock the achievers cape.