As a sorta endgame I would suggest this:
A good alliance is formed and it begins to research a way to defeat the player. After X turns they unlock a ritual, which will either just defeat the player completely or just for a time being. Turns to research a ritual are determined by the number of scientific PoIs in the alliance. Ritual casting also takes time, but it is much less and is based on the number of religious PoIs in the alliance.
Pros:
- Gives players a way to counter it by shadowing or outright destroying Places of Interest.
Cons:
- Effectively just a timer, which can be disabled quite easily
After the ritual is done, the player is either
1) Outright loses the game and is forced to start a new one
2) The game "skips" ahead in time either by passing a lot of turns or by deleting a bunch of nations (70%?), spawning new ones and regenerating all nobles in the remaining nations.
Also, how about name-specific endgames, which disable one of the names? For example, humanity creates an alliance and it:
1) Destroys Deep One spawning pool, effectively removing the ability to spawn more deep ones for a long time.
2) Enforces usage of post-combat religious rituals, which disable the ability to lower the world's temperature.
3) Creates an anti-flesh poison to defeat flesh growth with ease.
And so on.