I like the idea of Sophons experimentation.

I found this very interesting and I really agree with your take on how these races would react to such a quest when confronted with it.

I do find it interesting, however, that when playing the Vodyani -not paying much attention to the lore- and trying to follow the "Religious" paths presented (or at least what I thought helped the regligious groups) I end up with the siding with the Heretic (basically the path best for Vodyani growth and power, especially early game).

You'd think that it would be a bit harder for them to side with the Heretic if that weren't their natural tendensy.

But super fun to think about this!  I'm gonna try my next game with more roleplay in mind!

I think every race have pros and cons of joining either side. For example, Unfallen: on one side, they can try to bring other Lost on behalf of their own creator, but they can choose to not to bring them, because of imbalance this would create or Lost not really on good terms between themselves and return of them would proove dangerous for their master. Vaulters too, they can choose between honouring their legacy as Aurigians, or protecte their future from new unknown force, which Lost would truely be.

So do other. For the entire game, as I know, we knew only 2 of Lost. One died with Auriga, and one gave birth to Unfallen, a powerful race of cosmic caliber. Who knows what their return would bring.

As others have pointed out, there is no "canon" choice, only some choices are more likely than others, but you could also factor in the current galactic state when making that choice, for example, factions would probably want to side with their long term allies given this very important matter, and they might bring in new arguments for or against the return of the Losts. They could even have a change of heart at the last moment, take the vodyani for example, even on the Heretic path of their main quest, they still get asked which side they wish to join.

Ultimately, I think the fact that there is no continuation apart from a small dialogue change in the faction outros means that the writers wanted every player choice to be canon, regardless of any predisposition for one choice or the other for a particular faction.

For the sake of interest to the story and to future Lost-related content (one can hope?), I think rejuvenation outcome should be canon. I mean, what was the point of foreshadowing their return for so long and in so many media, and then just going "nope, not going to happen" route?

Their vision. Your future.

I know the way that Amplitude like to talk about the Endless games' narrative is basically "they're ALL canon," so basically whatever decisions you make in a given playthrough are what happens in that universe. They're intentionally very dodgy on what does/doesn't happen in the lore, because of that. The only time they ever went through with it was the glazing-over of Auriga, and the Vaulters story. Since there aren't any game mechanics that require certain choices (yet) in Endless Space 2, they're all correct.

I do think that we're gonna see the Lost in games eventually though. At least, with more of a presence than their current nebulous godlike influence.

I've thought about this many times myself and I came to more-or-less the same conclusions you have.

I've always pictured the Umbral Choir as a little more pro-rejuvenation though since the death of the Endless sparked the "suffering" that drew them to the galaxy in the first place and the restoring of the Lost would largely "heal" the galaxy and restore its balance and harmony in their mindset, hopefully restoring their own shattered harmony in the process. I think the decision to keep things as they are requires a little more mental gymnastics on their part and would be more an act of desparation than a conscious choice that it was what was best for the galaxy. 

I'm pretty which side Horatio chooses depends entirely on whether the Heretic committed some unspeakable faux pas when he contacted Horatio with his offer.

"I will NOT side with anyone who has the poor taste of wearing white after Labor Day!"