One of my favourite ideas for D&D 3.5 was having the adventurers interact with ghosts.
The Ghostwalk book spawned a short series of adventures within a campaign, where the players find themselves waking up in a ghostworld filled with extinct animals and peoples, and trying to escape back to their lives.
I'd love to try that again with a new game. Have the players do a few short sessions then suddenly come up against a nasty big bad guy. With a swing of his sword, he fells each of them.
But then they wake up in the world inside the sword.
Partly inspired by the sword Dragnipur in the Malazan Book of the Fallen (which I've written about before), and partly by a Daedric quest in Skyrim, the characters would have to advance within the sword, find a way to break out, and then avenge themselves and any friends they might make whilst imprisoned.