So, I've been developing this theory for awhile and I think it'd be pretty cool if it is true. So my theory is that the timeline of Adventure Time, after the apocalypse is essentially engraved in the universe. In other words, it's "destined to happen". Let me explain.
In Finn the Human, Finn wishes for THE lich to have never existed. And yet the bomb still goes off, an insane Ice King still arises, and a lich like being is formed from Jake. So my theory kind of revolves around the same idea as Bioshock Infinite (spoilers): "There's always a man, there's always a city, and there's always a lighthouse". In this case, there's always a lich, there's always a bomb, and there's always an Ice King. Finn wishes for THE Lich to have never existed, but A Lich still arises.
So I think that if we were to continue watching Finn's altered timeline for a thousand years we would see that Finn Mertens would eventually evolve into the Ice King, receiving some of the memories of Simon Petrikov from the crown, Finn's little sibling would go on to continue the human race with other humans underground, eventually leading to the same Finn we see in AT, and Jake would eventually become full Lich. In fact, for all we know, in the AT timeline we know in the show right now, this could be exactly what happens. Perhaps simon still froze the Earth and Finn Mertens eventually became Ice King, starting the apocalypse and unleashing the Lich. Ice King is unsure of his memories, possibly because they aren't even his. The only part I'm unsure of would be Marceline. I mean, she did die in the Finn Mertens timeline so that can't be what happens, unless she is actually our Marceline's mother and young Marceline is in the Nightosphere, only to be abandoned on Earth after the apocalypse to see the destruction. Finn Mertens, now with a beard and glasses because the crown has impaired his vision, turning him into Simon, stumbles upon the crying Marceline. He tells her his name is Simon, believeing himself to be so. This would explain the non-existence of Marceline's mother. In fact, this would make more sense because Marceline is seen to be very young after the apocalypse in the flashbacks, so what if the Marceline we saw in Finn the Human was her mother who died in the well of power. Young Marceline (the one we see in the AT universe)'s father abandons her on Earth. She is later bitten by a vampire. What do you guys think?