So not too hard a thing here, just to make hardmode variations of all the PHM potions. So instead of ironskin potions, you get mythril skin potions. Change an iron ore to mythril, maybe add a soul of night or light, and have to use an alchemy table instead of the standard bottle on surface. Mana regen could require pixie dust. That kind of thing. Maybe even make them last an extra minute or two, depending on the potion.