So thoughts about the future in Adventure Time
So here's what we know: a thousand years in the future the Candy Kingdom is abandoned
Princess (Queen?) Bubblegum is missing, Marceline is presumably searching for her. The Candy people are being held in stasis in the Prizeball Guardians.
According to the comic books, in 1000 years Marceline is the Empress of Ooo
But even here she's living in a derelict, abandoned Candy Kingdom without Candy people in it. Off screen there was apparently a second Mushroom War, and a shot of the earth shows it even more blown up than it was in the current timeline.
Kind of hard to say exactly what happened.
Here's what else we know: Charlie's son, Gibbon, rules the earth with the power of one of the gemstones from the Ice Crown, stolen not from Ice Thing, but from Turtle Princess. Cause remember, Ice Thing gave that gemstone to his wife for her wedding ring.
So what happened? I think it's safe to assume that at some point in the future, the humans that returned to Ooo at the end of Come Along With Me got into some kind of friction with the other inhabitants of Ooo and Dr. Gross caused the second Mushroom War and destroyed everything again. Plain to see in the Season 11 comics that they were going to be a problem.
Either Bubblegum or Peppermint (because he is shown at one point being in charge of the kingdom, so it's actually up in the air on who) created the Prizeball Guardians to protect the Candy people from getting destroyed in the coming war. The Prizeball Guardians go off into the world to protect the citizens, only to be shut down in the future by their creator presumably once they are no longer needed. Their creator has vanished and now they can't be shut off, so they wander Ooo doing what they were programmed to do; protect the Candy people.
At some point during or after the second Mushroom War, Marceline and Bubblegum come together to lead the survivors, hence why Marceline is the Empress now. They were probably planning to release the Candy people once the danger had passed.
This enrages Gibbon for some unknown reason, he perhaps thinks that they're not fit rulers in the resulting wasteland. He probably even confronts them about it.
Either Turtle Princess is killed in the war and Gibbon steals the gemstone from her body, or Gibbon kills her for her gemstone. Whichever way you choose, it's a power grab. Either direction, this action causes Ice Thing to lose his mind and go feral.
Ice Thing kidnaps Bubblegum and Marceline goes out to rescue her, leaving their thrones empty and allowing Gibbon to assume control of Ooo and begin to steal the powers from all of the other Pups on earth using his stolen gemstone.
Which leads to Shermy and Beth being little rebels and trying to save the world from Gibbon's tyranny.
On the other hand you've got Penelope and Betty, who are possibly out searching for Bubblegum? Because a lot of people headcanon Penelope to be Bubbline's child, so it makes sense that she'd be out there looking for her lost mom.
But, you ask, if Shermy is Finn's reincarnation, but so is Penelope, how can they be in the world concurrently?
Because Finn canonically gets his soul split into separate bodies.
Finn and Fern are the same soul in two bodies. Shermy and Penelope can both exist at the same time because Finn's soul is split now. I don't think it's outside of the realm of possibility that this could happen.
So then what? Penelope and Marceline somehow manage to save Bubblegum and return to Ooo for the final showdown against Gibbon? Shermy and Beth manage to get the gemstone away from Gibbon and that summons Ice Thing to come and take it back, leading Shermy and Beth to Bubblegum? Marceline and Bubblegum return to the shell of the Candy Kingdom and try to return to their lives before all the mess happened?