Campaign Ideas

General

Have newly created characters be connected to the previous characters in some way.

(Idea from Flintwyrm)

Each time the world ends, add more content and classes. Optionally, have the content and classes be dependent on how the world ended.
Optionally, have a graveyard for dead characters which the players can choose from when making new characters.

Have two counters for mission/session success and failure. The definition of a success/failure is up to the GM’s discretion - for example, a TPK can still be a “success” if the mission objective was achieved.
7 successful missions prevents the apocalypse.
7 failed missions brings about the apocalypse.
Do not roll the misery dice per in-game day - instead, for every failed mission, add a misery - preferably fitting to how the PCs failed. (Failed to stop a necromancer/lich? “3:1 At Graven-Tosk the soil shall grow warm and those who rest be made to walk.”) These are announced at the start of the session following said mission.
Mission difficulty should (roughly) scale with the number of successful missions. It should go from kinda tough to insane.
For every 2 failed missions, allow the players to Get Better once during character creation. (For example, 5 failed missions allows new characters to Get Better twice)