I really like these ideas. The NPCs already try to form alliances towards the end-game, so it makes sense to give one of them the title of "Kingdom of light" or whatever, and then have them begin work on some anti-dark ritual. The idea of it having a set number of locations you need to target to stop the ritual is also good, should let you work against them politically by shuffling a few key broken nobles around, agent-wise by targeting those nobles for enshadowment, or militarily by wiping them out.
The concept of having a "retry in 500" years is also great. It should be possible, with the current setup. Might be a few technical difficulties (saved games need to avoid getting too huge if they record everything), and gameplay ones (NPCs need to die of old age, so you don't get the same people 500 years in the future). Other than those issues, it's already possible to make the game play hundreds of turns to generate a history, so we could just delete all monsters, reduce shadow over time, then play forwards the game. Maybe you'd even have a tiny dark kingdom which managed to survive this age of light. If we can get this to work, we should also put it into the victory screen. Might be possible to get this in the next version, let's see how things work out.
Specific defeat conditions are also cool, but that'll have to wait a version at least, to keep this version from growing too huge, and leaving too long a gap between v16 and v17's releases.
Thanks for the suggestions, hopefully we can get something together which looks like what you were imagining.