I'm playing on hardcore mode, and I have to say, it's not that hardcore
:I
If you don't think it is, Then notch should do:
Make the mobs spawn rate very high even at night.
Mobs do more damage then hard mode.
Creepers explode 2X the size and 4X the size when charged.
Charged creepers are common.
Falling does twice the damage.
Drowning does twice the damage.
On fire does twice the damage.
No mobs will burn in the day.