I definitelly don't want the boring "multiple paths to victory with a single victory condition per type" thing :)

So, I was thinking about something along those lines:

There are achievements (own X planets, have X prestige, all major races acknowledge your superiority, etc), you either got it or not. There are "disasters/problems/troubles" you either solved it or not. Now, in order to win the game you need to claim 3/5 achievements and solve minimum 3 disasters.

Examples:

Achievements:
* prestige at least 1200
* prestige at least 500
* control 200 planets
* control 150 planets
* control 100 planets
* all civilized races acknowledge that the Empire is a major power
* all major races acknowledge the Empire's superiority
* have more vassals than all other major races together (but minimum 2 vassals)
* seal 50% of trans dimmensional rifts
* build a star fortress
* there are no rebel worlds
* research 20 technologies from the sociology field

Disasters:
& solve the conspiracy plot
& defender of the galaxy (fend off trans aliens invasion)
& the need for imperial reform
& suppress the rebels

I'm not sure :D Go for it? Change it? Adjust it?

Perhaps the "fulfill X other conditions" could be an option that sits alongside the player being able to select any one of these options as their win condition? For games like 4X which tend to suffer player boredom and abandonment far before win conditions are met, giving the player more control over those win conditions is a good idea.

Or for simpler setup, you could have "easy, medium, hard" with different numbers. Easy = Fulfill 1 condition, Medium = Fulfill 3 conditions, Hard = Fulfill 5 conditions.

Either way, I definitely like the idea of having to "collect" win conditions out of a set. I'd say go for it on that score.

For the disasters, my only reservation is: are the "disasters" present from the very beginning of the game or is it possible the player will have to wait around for them to come about? I'd say that surviving a disaster should simply be rolled into the set mechanic; ie, surviving a disaster counts as an achievement and counts towards your total, rather than being a separate set you need to collect.

You are right, disasters should be merged, there is no need to treat them separately.

OK, like this then: you have victory points (achievements in the first post, each count as 1 point). When you collect 8 victory points you get an option to be crowned the ruler of the galaxy (which would take several turns and you need to keep the victory points valid during that time - you can lose victory points if the condition at any point is not met). The "crowned" thing I think is needed to avoid lame "sudden victory I did not expect", instead you would get "you have collected X victory points, do you wish to start preparation for being corowned the ruler of the whole galaxy?"

There could be more victory point unlocked later I suppose. Like your advisors get an idea for a cool monument and from then on you are allowed to pursue that goal (possibly also a mutually exclusive choice "do you want condition A or B?" where one would be militaristic and the second pacifistic for example). Or technologies could unlock some "construct AI network for 1 VP".

It might feel better if you have to complete those achievements within limited time (it can be a long time, but still a clear limit)

Perhaps, you also need to survive until that limit, even after completing the necessary achievements.

So you have to do some time management as well. Maybe you can get all the achievements by going all in, but expend too much resources doing it and fall before the time limit. Or maybe you want to play it safe and leave a difficult achievement toward the end (eg build something big which requires selling most of your army), but something unexpected slows you down and you lose.

The coronation thing is good - you're right, sudden endings are kind of annoying, even if it's a supposed victory. Especially in a game like this (which I expect will be more about creating an evolving story than mechanical achievements).

Maybe there could also be "negative" points that are difficult or even impossible to remove? So you could have 8 points (the win number) but also one "negative" point from something you did or failed to complete, thus having a net 7 points and not able to crown yourself yet.

Trying to think of what negative point things would be though. Something that harms your prestige and standing - maybe having been found out in a murder plot you instigated, or similar stuff. Things that would cast your character/dynasty in a worse light.