Okay so obviously the most important plot element in this episode was the PINK BLOB, which I will affectionately refer to as ‘The Source’.
Before starting, I’d like to kill to kill the notion that that benevolent pink blob was Princess Bubblegum. Here is the abridged version of why that’s not possible: PB isn’t that old- she has ancestors, rulers that preceded her, and lacks knowledge of events that occurred even during the time of Billy. Also their characters are not alike.
What is The Source? I as well as many others have hypothesized that it is some type of proto-candy person, perhaps the common ancestor for all (non-synthesized) candy persons, or just candy royalty. It’s quite possible that due it’s enormous size (it stretched across multiple city blocks) that it served as the building blocks for many other candy creatures. We’ve seen that candy biomass is non-exclusive and easily transferable, as in Too Young PB ages using bits and pieces from many different types of candies. PB’s experiments with the little farm people in the Lich, as well as Lemon Jon’s ability to dissolve the bonds uniting him demonstrate that even an individual candy person can donate their component to create new life or non life. Because of this the source is not necessarily made of or always made of bubblegum, and at any point The Source may sculpt portions of itself into other beings or structures.
A question that arises is how did The Source become sentient? Some may contend that it’s due to radiation (boring), but I submit an alternate hypothesis. The Source never displayed any level of intelligence until after it engulfed Simon’s backpack. Is it possible that he was carrying the Enchiridion (which at this point still contained all the gems)? If so, perhaps the magical gems possessed the ability to grant cognition and motor capabilities to non-living matter. This would be cool for any number of reasons, but there is no way to confirm it. The implications would be that Simon is more or less the patriarch of the Candy Kingdom (among others), and could potentially offer another explanation as to why all of the gem holders are known as princesses rather than queens- The Ice King is Lord over them all.
So if The Source is relatively immobile, it is quite likely that the town in which the episode took place is the present day Candy Kingdom (for some weak evidence compare the can of grape soda found on the ground to the ones from Death in Bloom). With that in mind, consider what Marceline said after they captured the deer: “If there are deer in the forest, do you think there are other things?” In Susan Strong PB mentions a certain forest where her Uncle Gumbald cut down all of the trees, a forest that contained the hatch for the bomb shelter the Hyoomen were hiding in. If a deer managed to survive the apocalypse unscathed, who is to say a human couldn’t have? Marceline’s lines may offer a foreshadow of the Hyooman species, or even Finn’s ancestors (probably but not necessarily mutually exclusive groups).
For all we know parts of The Source may be tucked away in some secret room inside of the Candy Kingdom’s castle, either ready to be formed into a new leader, or merely as a tribute to the heritage of a people. Perhaps its spirit lives on to this day, its influence still seen and felt throughout the kingdom.
The amount of possible speculation is immense, so I’m not going to even try and cover everything this blob could or could not mean. The most important thing to grasp is that it’s a precursor.