Possible explanation for Mantid's backstory:If we go on the assumption that the Brothers' weapons of choice have to do with their previous life, then Mantid was definitely a spear-wielder — probably a disposable, light infantry soldier, since those were the people who most commonly used spears (though he could also have been a master fighter or a forger of spears, since he has so many of them). He was probably killed by a spear strike as well, as evidenced by the spear through his torso. However, it's interesting to note that the spear is oriented as if he was stabbed In the Back. Perhaps this indicates that he was killed through a betrayal?
The Scorpio are the denizens of a Limit that has fallen into the NightmareThe genrally rotten look of the Gnats implies they died. And The Brothers rose from the Nightmare, and became the victims of much Body Horror because of it. Therefore, Everyone in The Void, except the one who asceneds, becomes an entity like the Scorpio.
Ino split on death into GLaDOS and the Nameless Sister.We know that Ino reached breakthrough at some point, but she apparently also got killed. The Nameless Sister is apparently either dead or extremely close to it, until she gives you all her color for the second time and dies even more. And then there's those Companion Cubes lying around. This can be explained by having the Brothers sniping her in the middle of her ascent, causing her to break up as she was leaving. The part that made it became GLaDOS (Her era wasn't expressed properly due to being overwritten by Caroline and possibly the Combine invasion, or it did and it wasn't a good one), the part that didn't fell down and appeared dead and out of color as Death the Nameless Sister. However, either because there was a gold or emerald color plant within reach, the method that the Nameless Sister gets more color somehow on occasion kicked in by the cycle, or some other factor, she managed to not die horribly and escaped to the security of her island. The greenhouse may be bombed-out and the willow near death as a result of what the Brothers did to keep out Ino. Meanwhile, GLaDOS is pissed about her exit from the Void as Ino being so rudely interrupted and between killing people with neurotoxin and turrets and playing Portal testing chambers, killed a ton of additional people in order to cause the player to appear because given sheer force of spamming dead people luck would eventually cause one to get caught. She either can detect what the hell the Nameless Sister is doing and possibly has some method of influence over her, or through killing lots and lots of people can break her way into the lower Limit that is the Void in order to deposit companion cubes as a warning message. Either she's out for vengeance or just doing it for science, but she had some plan if the Void didn't implode.
Yani ascended before.This is why Yani claims to have been to the Surface before: She actually has, but her era came to an end, she died, and she returned to the Void.
- A similar process may have happened with Ole because her flat is similar to the overworld and she can see the limit above.
Godsend is an ascended Sister or similar.Godsend is a known inhabitant of the Surface who is visible to those in the Void, who happens to remember the Sisters down there and sends messages via the Adit, dreams, Uta, Yani, Ole, etc.
There used to be tons more color, Sisters, Brothers, chambers, etc.This is how despite Breakthrough using the entire Void's worth of colors, there were apparently enough Sisters to have 40 of them dead and one for every major era of mankind ascended, with the current cast showing up. Color had a much stronger grip a long time ago, but Breakthrough after Breakthrough left Color severely depleted and then the player's existence caused it to degrade severely and finish it off.
The Void usually regenerates when destroyed.After killing everything inside after a successful total Breakthrough draining everything of color and plunging everything into the Nightmare, it grows back with a new set of Sisters and chambers popping into existence by drawing Color or a similar resource from the limit or worlds above. However, since the Surface is apparently undergoing a great hunger worse than the Void, it's either prepping to do the same and can't provide any Color at the moment and the Void will just have to wait, or is actually dying and the Nightmare is going to make a big jump up the Chain.
Eli's blimp actually could fly someone to the next Limit.It sure doesn't look like she actually filled it with Color and tried to fly away as she warns the player against, nor would a Sister in the middle of a Color-drought-induced famine think that's worth wasting Color to test. However, everyone on board would probably go insane due to passing through the areas between the connections to chambers and the end result would be useless to anyone besides Eli and maybe someone else in the chamber. She is also sitting in the entryway, preventing us from trying for that matter.