Minecraft Hard mode (within the game)

To increase the difficulty post end game, through a quest system similar to the Ender Dragon. 

First you'd need the Bad Omen Potion and make it into a splash potion. Next you would need a beacon that was at top level and splash the potion onto it to enter Hard mode. Given that beating the Wither is one of the toughest bosses in the game, creating these conditions would mean you are already at end game. 

Hard mode would allow for mobs with armour, and with enchanted armour, to spawn far more often. Charged creepers would spawn naturally, maybe 1 in 5 creepers. Raids can happen within your home, based or based on where your bed spawn point is. Wither Skeletons spawn naturally in the overworld, and with armour sets as well.

This is more of a foundation than anything. Given that all suggested would simply be changes in spawn rates, and loot to armour, I think this feature could be added within a Minecraft Drop, even as a side feature. 

Extras:

Above would be what I think should be added, but here are some extras. 

Additional wither based enemies. Given it involves the Wither star, you could maybe have wither variations of zombies, villagers, spiders, Golems e.t.c, hostile to the player. Stronger, but more difficult trades. 

This would also be a good experimental ground for Mojang to test difficulty or add bosses, without disturbing traditional minecraft play.