If you check out my February 25 post, "ABC's Lost - Is Jacob a jailer?" it seems to fit in with the events of last night's "Ab Aeterno." (Spoiler guesses ahead!)
In the Richard Alpert-centric episode we went back in time to learn his story. He did indeed, as many had guessed, come to the island on the Black Rock ship as a slave in chains. His story began in 1867, which makes Richard and Jacob REALLY OLD GUYS. If you want to read my entire recap, check it out Sci Fi Wire.
At one point Richard (who is Richardo then) sits on the beach while Jacob describes the bottle if wine he's holding as if it were filled with evil, with the cork being the actual island. My guess, that the island was a jail and Jacob was Smokey's jailer, seem to apply quite nicely.
At another point the Man in Black talks to Jacob about the fact that he's ready to kill Jacob to get off the island. And Jacob reminds him that if he manages to do that, someone else will take his place.
What this all means to me is that the Man in Black is an evil of sorts that must be contained. Since Lost seems to be about balance, maybe he's the evil in men that tips the balance to the bad side. If that holds then when we take a look at what happened in the Sideverse, where Jack and his crew successfully sunk the island, where is the Man in Black? It's unlikely he died, which means he's out in the world waiting to do his damage, to tip that balance.
So my best guess as to what's really going on... and again stop reading if you want to be surprised and I might just possibility be right... AGAIN STOP READING ... I MEAN IT ... I MIGHT JUST BE RIGHT ... PROBABLY NOT, BUT MAYBE ...
My best guess is that nothing matters in the Sideverse because the Sideverse is about to die in one way or another. The evil that was let loose when Jack and his friends blew the island to hell, got off the island and out into the world. So while Jack has a son in the Sideverse, Claire's still pregnant and innocent, Locke is alive and engaged, etc... the world as they know it is about to come crashing down around them destroyed by the Man in Black and his evil.
I'm sure others have come up with this scenario. And once the Sideverse is gone it will be all Lostverse, with no more flashbacks, forwards or sides. Just good against evil, and a battle where most will die, a couple may leave the island, and at least two will live trapped forever in an endless battle.
Can't you just see that chat on the beach from last season's finale between Jacob and the Man in Black, except now with Jack and Locke. How Locke, the Smokey/Man in Black, hates Jack for keeping him trapped on the island and Jack saying until it's over, it's all progress. In the meanwhile, a ship or a plane buzzes by and we know it's a continuous battle between the forces of good and evil.
That would be a pretty good ending in my book.