So, my main point is: I think that turquoise goo creature seen briefly in the Mouthful Mode reveal trailer is the actual main antagonist.
There's a scene in the trailer where Kirby is in a skyscraper rooftops environment looking up at something, which looks quite different from world 1 and also has a sense of finality to it (and it doesn't look like part of the volcano area either,) and the scene with the goo creature might be the same area due to the cloudy lighting seen through the holes in the roof there, but we'll see.
Furthermore, I highly suspect that the leader of the Beast Pack is not the final boss and that we will come face to face with whatever eliminated the Forgotten Land's original residents. It would need to be able to do that without seriously damaging the buildings. and a goo creature could be capable of that. It's also the same color as Elfilin's fur, and I think Elfilin probably has something to do with the final boss even though I don't think he's evil (but I can't be 100% sure,) so yeah.
So, here's my guess as to the rest of the plot:
Many years before the game takes place, Lightron Works Co., which was involved with space exploration, found a spacetime rift and recovered a small turquoise creature from it. The creature contained a great deal of indescribable cosmic power within it, but, furthermore, the energy inside the rift had caused its body's form to become quite unstable. (You saw what happened to Kirby in the intro cutscene when he was exposed to the same power.) Some people there wanted to experiment on the creature more, but others didn't, causing Lightron to break up into multiple smaller corporations. Nevertheless, some former part of Lightron kept experimenting on it, causing its body to become completely unstable and turn into goo. Having been driven mad by having been experimented on, and also unable to control its instincts, it went on a rampage through the city. Sometime after or during this, the moral part of its consciousness realized the sheer atrocity of what it was doing and split off from it, becoming Elfilin. (The fact that Elfilin is the creature's separated heart would be why Elfilin looks like a heart in the first place, and also why Elfilin's fur is the same color as the goo.) The rest of the creature shrunk way down due to having just lost a great deal of its power.
Eventually, nature would reclaim the island. The Beast Pack would also take charge of the island; either they arrived on the island's shores in search of a new home, or they happened to have already been there but were spared by the creature. They would prosper here, but, at the same time, they lived in fear of the creature, which would demand sacrifices from them in order to satiate its appetite, and also demanded that they help track down Elfilin. (I assume that King Dedede here is this dimension's resident Dedede, given his distinct design. There is also the fact that his clothes are very tattered, which implies that he had been in this world for a long time.)
Elfilin had a plan to defeat the creature, though: he would build up cosmic power, and would use it to create a portal to summon a hero from another world who would be able to face his adversary. He very greatly underestimated his strength, however, and ended up bringing a large number of Waddle Dees to the Forgotten Land in addition to Kirby. The Beast Pack went to work capturing the Waddle Dees as they would be easy sacrifices for the creature. Elfilin felt personally responsible for this, which is why he is so personally invested in helping to rescue the Dees. I don't know why Elfilin just doesn't tell Kirby about why he is here, though. Perhaps Elfilin doesn't want to distract Kirby from saving the Waddle Dees first, or maybe Elfilin got bonked on the head by some falling debris and forgot.
In the end, Kirby goes to the Beast Pack's lair in the volcano and beats up their leader to rescue Elfilin, whom Dedede had captured. But, the Beast Pack's leader tells them that they must defeat the creature in order to end the strife between the Pack and the Waddle Dees, so they head to the highrise area. After some tense platforming, the creature kidnaps Elfilin, Kirby fights it on the building's roof, the creature absorbs Elfilin and becomes huge and powerful, and Kirby does some kind of insane Mouthful Mode to engulf a building or something to effectively also become huge, and so there's some kind of weird over-the-top kaiju battle.
Maybe, after Kirby kicks the creature's butt, it and Elfilin have some kind of reconciliation with each other, shrink down to a reasonable size, and become the proper guardian of the Forgotten Land, and this design is what they'd use for their future appearances in other Kirby games. But IDK.