So, in this AU, it's set after a point both Time and Sabre return to the timeline.

They don't know what it was, but something started destroying the Realm entirely. No one could figure out a way to stop it at the beginning, although it didn't seem like much of a problem then. Suddenly, things kicked up, and soon everyone in the Realm was gathered in places around the Overworld. The World Beyond was destroyed, so was the Middle World, and most of the planet. Everyone tried to find a way to stop this, even Origin started fighting against it, but not even he could do anything. They knew they were running out of time before everything was erased.

While trying to figure out something, Sabre revealed something to M and the Professor; He and his people were re-engineered through experiments to be able to contain the souls of possibly millions, and transport them to new worlds in case of emergency using their natural ability to travel through dimensions/universes, being resistant to the damage of doing so. However, they all hated the experiments, and escaped from the lab they were in. Sabre was separated from the escaping group, and then arrived in the RQ world. 

Sabre tells them that since he was rebuilt to hold millions, if the need arises... he could contain the population of the Realm entirely and find a new world for them to settle in. That includes the spirits, and many other outliers. The people could even take items and supplies with them, in order to re-establish their communities and hold onto things precious to them, and also take animals like pets. He knows that it's possibly the only way to save his friends, and is willing to use this power to do so.

Long story short (and for drama purposes), it does get to the point that this plan must be used.

Locked into a machine inside the Professor's lab, Sabre braces himself for what's to come. Around what's left of the Realm, everyone also prepares themselves for the shift from normal to soul-trapped, holding close what's dear to them. As lightning strikes everywhere, the Professor remotely activates the machine, taking one last look around his half-wrecked lab. In an instant, along with a bright glow... the world is empty, except for one.

The World Below is able to break because of the lack of a shield around it now, and Sabre is running from the ones that are being left behind. A portal appears, one planned to be opened at that moment... however, the ground is breaking. Pieces of land float up and crash down, an abyss the only thing seen below. Sabre is able to jump over the gap, reaching the portal.

Before going through, he takes one last turn towards the charred beings, and sees a familiar red-eyed, cloaked figure. They stare for a moment, one knowing he has completely lost and is paying for his actions, the other knowing he has won and can move on to a new start.

Then, Sabre smiles, standing in front of the portal... right before flipping Void the bird and jumping through.

One world ends, and a new journey begins. One soul, once again looking for a new home... except he's now joined by a few thousand other souls within.

Everyone is conscious and aware, inside a place within Sabre's soul meant to carry all of theirs. Of course, the spirits and living are reunited with each other during travel, and both mingle as the journey goes on. You can imagine the fluff potential here.

Sabre can also hear everyone, although he can deafen himself from their individual conversations and such, even making himself unable to hear them entirely. Sabre always does this when he needs to sleep, as everyone isn't really using energy while being transported.

When I say the spirits are there too, I mean all of them. Rainbow, Galaxy, and many other favorites are there... that includes ones from SL. The Professor gets to see his ancient buddies again. Man, the meetings of the Giants and the RQ Steves are gonna be both awkward and kinda scary. Oh, and Sabre finding out he has a lookalike that's his complete opposite personality-wise? Hilarious. Hol' up, Rainbow is gonna find out that the crystals are technically made of dead people-

Hypno is there, too. Because of course he is. He teams up with Shadow to annoy the crap outta Sabre.

Speaking of Shadow... the RQ people learn of what Sabre had to deal with when Shadow was a problem. I can imagine that Shadow is being annoying and everyone just yells "SHADOW, SHUT UP!"

Now, about the actual adventure to find everyone a new world: Sabre will be traveling through different universes, looking for portals to go to the next one. I imagine that the worlds could be different modpacks, like CrazyCraft or RLCraft, you probably get it. Maybe they even stumble into some apocalypse worlds. Sometimes they could come across a normal world. The Steves would find out about Sabre's individuals skills and the tricks he uses in combat and such (basically our ways of fighting), and about items such as Ender pearls and elytras. They also find out about the many questionable things he does for fun... like fighting Withers or exploring the Deep Dark. Sabre will also make use of his parkour skills to get around.

Of course, you can't really have a book without conflict. The people that originally turned Sabre into a vessel-thing find out that he's using that ability, and try to track him down to both get their experiment back and find out exactly what the Steves are. This leads to many chase scenes, and maybe even world speedruns so Sabre can escape a certain world to get those people off his tail, using both his normal skills and getting some help from the Steves by them using their powers.

And uh... angst scenes also have many potentials here... since they're within his soul, it makes since that certain people could look through Sabre's memories >.>

I will leave what world they eventually settle in up to the decision of whoever decides to claim this :)