An idea for what could go in the building the the terrace which only has a roof entrance. Alternatively, it could be an unobtrusive cave hidden the Death Valley.
When Link enters the Ghost house, it appears that there's just a single, unfurnished room. Broken furniture is strewn about, and an empty frame hangs on the wall. However, using the shadow medallion reveals a hidden door that, when using the medallion, Link can go through.
On the other side, it's just a small sitting-room. There's a fire crackling in the fire place, a large, eerie portrait of a man on the wall, and an elderly figure wrapped in cloaks sitting in a chair by the fire. Link goes and talks to the elderly figure. "Ah, back from the store? It took you a while. What had I sent you to get? Oh yes... one dozen ghost souls." The figure giggles. The fire blows out, darkening the room. The eerie portrait on the wall changes from the man to a skeleton.
If the player hasn't encountered Poes yet, the old figure just sounds crazy. There's a treasure chest in the room with some rupees and that seems like maybe that's all that's here. However, if you've encountered Poes before, you can collect their souls after defeating them (like you collect Chu Chu jelly.)
If you have 12 Poe Souls in your inventory, the old figure will continue after giggling. "You have them? Well come along, let's do our chores." The figure giggles again and disappears in a puff of smoke and the eerie portrait of the man fades away, revealing a doorway behind it.
Behind the doorway is a mini-dungeon with just a few rooms. It's full of ghost-type enemies (Poes, Redeads, etc.) and creepy traps (trap doors to fall through, those false doors that slam down, etc.), the Shadow Medallion will come in handy.
At the end of the dungeon, you encounter the cloaked figure from the sitting room again. It's levitating in an empty room. The door closes behind Link and the cloaked figure transforms into a miniboss or strong enemy. Defeating it will cause a chest to appear with the Quartz Talisman, which when acquired slows the rate of magic consumption for the Shadow Medallion.