After beating the game, you will now be able to play new game + with all of your inventory and level from the past game. However, you won't be able to change the difficulfy mode of this new game. If you beaten the game in 'normal' mode, you won't be able to change it to 'hard' mode in new game. On the flipside, creating an entirely fresh new game (this time from the main menu) does keep your unlockable outfits and weapons, but strips out all your player level and inventory to start from scratch.