Hard Mode: Run the shop inside the dungeon to resupply adventurers passing through.
Advanced Hard Mode: As above, but leave enough monsters that your customers will be more-or-less depleted by the time they reach your shop.
Inferno: Run a successful shop in the abyss.
Nightmare: Run a successful chain of stores with a presence on each level of the abyss up to the 666th.
Insanity: Run a successful shop in the Far Realms of madness.
New Game Plus: Start a new game at level 1, and eventually buy out the entire on-hand inventory from your old character's Far Realm store. Without dying or going insane.