Something I think that might be an interesting crown store item would be themed music packs. Since this is purely on the player's end they could be anything from ambient styles to heavy metal depending on what the player wanted to purchase and use. These packs could include exploration, dungeon, boss fight, combat tracks, etc. Once purchased then the user could change out their music styles however they want. Right now I always leave the music off because I have heard the same tracks for years.

Outside of ESO, ZOS could accept submissions from artists and handle the licensing and compensation part of it. I'm sure there are a lot of excellent musicians out there and their name could be listed along with the music pack.

"Oh look, the Malukah Skyrim music pack is available now in the Crown store!"
"I'm heading into Cyrodiil, I want something metal!"
"I want something ambient and relaxing while i'm fishing."

Because this would be totally up to the tastes of the player, the genres could really be limitless.