When you beat the game you'll unlock each of the following new modes (in the following order):

Classic: classic weapons can only be built using bluprints. Classic aiming is enabled. Difficulty is hard. Co-op is disabled. Beat Classic to unlock...

Hardcore: you need to beat the whole game without dying and if you do die, you'll go right back to the start! You can save though. Beat Hardcore to unlock Retro mode.