What if you could access someone's memory palace? Find the magical phrase, use the proper ritual and you are transported to a structure. Each room might have odd objects, texts, and maps that the creator wished to remember. It might be a kind of mini-plane the players are actually transported to, or more an ethereal dream. I like the former even better, but would limit time in the palace to an hour a day and exiting would drop you back in the mundane world exactly where you entered it from.
Why visit there? It's the dungeon of a mind. You might find there:
- unique spells
- alchemical recipes
- secret treasure locations
- martial arts techniques
- battle plans
- secret trade routes
- faction relationship maps
- blackmail secrets (of others or the palace owner)
- serial killer burial sites
- Western March style maps of a region unexplored
- monster classification schemes, along with the monsters!
- daily routine (for assasinations)
The palaces themselves could be locations the players are familiar with. Perhaps the palace owner is very familiar with that Snake Cult Temple. Imagine how freaky it could be for players to revisit a dungeon they had cleared, or a tavern, or keep they are familiar with.
As a treasure item, allow a player to create their own palace, and only thus become an alchemist. Or maybe require one for the vast amount of data needed to research a spell.