Well.. there's a possibility that this might've been written about before, but I'll post my idea anyways just to see if there's anyone who might agree or disagree with it, because well, it's just an idea.
I've noticed that empires surrender themselves, like anyone else has, but I feel like if it was a sort of unfair ending for an empire.. they just go and surrender their two or three planets left (or whichever number) and then plain disappear. Obviously this means that if we ever surrender to an AI then the game ends because our empire just goes poof.
Then I thought, why should it be like that? Why not make surrender for human players work differently. I've for example have seen myself flooded by enemy ships at unexpected times because of some mistakes of my own, and only have one or two races that aren't at war with me stil. So yeah you try to fight but sometimes you just can't produce that much military wise. Or some other times when you're trying to go for a more economic/research game where you forget or begin building military power once its already too late, and watch your small efforts go to waste.
So, when that happens, why not being able to surrender to any empire like the AI does, but instead of forfeiting the game with that, make you a sub-empire of the race which you surrendered to? With this I mean, you, as race-x, stop existing. But you continue as "governor" for the few planets you gave up.. deciding what to do with its production (which ships to build, or improvements), and as "militar governor" too of the ships made within those planets (obviously going to war with whoever the main race is, without being able to go to war by your own) while still being asked by the main race to give them certain ship(s) which you'd have to do.
Maybe you could even add a diplomacy extra thing, where as you'd normally be voted whether your party stays in power or not, now its your party against the main race party, elections in which if you lose you also lose the game (considering that you'd no longer be governor).
Within the research part.. well not sure, maybe make your planets research as individuals too and continue with the same tech tree you had before surrendering.. or if you want to make it more complicated, split the tech tree, giving your techs to the main race, while also being able to research off their techs (but in case you split up, and lets say you researched the last tech of a weapon because the main race had the before-last one, it'd be disabled for you unless you research since where you were at the beginning).
Now you may see that I mention there a split up. Well why not let you as a surrendered sub-empire, eventually ask for independence again once you feel strong enough.. with three chances of fulfilling this. First "good" with your main race by just plain asking things or such.. if it doesnt work then the "neutral" way which might be based off giving tribute+tech+ships or something alike, and if not then the "evil" which would be declaring war upon that race which let you survive for an extra minute (the kind of "we're not ever signing a peace treaty" sort of war"), considering that you would probably not be as strong as them because of the constant ship/bc tributes. Maybe after independence declare your empire/race under a new name.. you know, just like if you evolved or something.
Of course you may say that it could be abused.. but maybe if you put in some limit.. either just being able to surrender 1/2/3 times (depending on the size of the map), or hurting your "trust level" for "surrendering" to other races after being independent once (for example, at a start you may surrender to any empire.. go for the drengi/korath so that they can protect you with their military; but then as you break thrust then only being able to surrender to the one or two races that may still have you in "close/friendly").
Maybe give you certain penalties like a permanent production loss because of permanent boycots and such..
And I think that's about all I have to say. Personally I find the idea of being able to do this very very interesting and potentially fun. Gives you the chance to survive even the worse of your games.. and I think it would certainly change how the game itself is played. Of course.. I don't expect it to happen or anything alike (even tho I'd take it as a very pleasant expansion update/gal 3 upgrade), but as I said, it's just an idea that seemed interesting enough to share and see the feedback from more experienced players.