Kirby's Return to Dream Land/WMG

Egg Engines is the remains of Mobius after Eggman finally won.
The entire planet is a mechanized wasteland populated only by machines in the shape of living creatures. The name sounds just like one of Eggman's creations. And the boss of the area happens to look just like something the fat man would build to kill Sonic.

Raisin Ruins will be the ruins of Grape Gardens
This of course would never be explicitly stated in the game, but it would make sense due to the naming and might be implied through the world's visual design. Similarly to how Shiver Star in Kirby 64 The Crystal Shards was implied to be Earth after nuclear winter.
 * While not stated upright, there is a remix of Grape Gardens present, although not in Raisin Ruins.

Spear will get a Super Ability version
Sword and Hammer already have Super Ability counterparts, and King Dedede, Meta Knight, and Waddle Dee are all essentially permanent versions of abilities, so...
 * Jossed.

Dark Matter and Zero will return as the true antagonist
Consider that many enemies from Kirby 64 the Crystal Shards and Kirby's Dream Land 3, that haven't been seen since the former, are returning in this game. Now consider that the game seems to be slower-paced like those aforementioned games, focusing on exploration and hunting down Plot Coupons. Also, Magolor will either turn out to be possessed by Dark Matter, get possessed later on, or will turn out to have been Evil All Along, with or without Dark Matter's involvement.
 * Jossed.
 * That is never confirmed, in fact it is more heavily implied that the Crown has a consciousness of its own (much like other Ancients' constructs, like NOVA and the Lor), and is simply manipulating Magolor.
 * Or,
 * That is never confirmed, in fact it is more heavily implied that the Crown has a consciousness of its own (much like other Ancients' constructs, like NOVA and the Lor), and is simply manipulating Magolor.
 * Or,

The alien is the final boss
An alien ship called the Lor Starcutter crash lands in Dream Land and pieces of his ship scatter throughout the land, so Kirby and his friends have to bring the pieces of his ship to him so he can return home. What did you expect from a Kirby game? For all we know, the gears which are sometimes hidden in alternate dimensions might be powering up the Doomsday Device.

Magolor was possessed by Dark Matter the entire game.
In his basic form, Magolor's body is hidden under his clothes, but he's clearly spherical-shaped, and sports a black body. His clothes actually hide Dark Matter's eye, while Magolor's visible yellow eyes are illusions. Now, flash forward to the final battle's second phase- Magolor's lower body is destroyed, leaving only his spherical black body, as well as his hands. The crazy grin he has is actually Dark Matter's eye- you can see the pupil when he opens it- and the eyes are again just illusions. (Similar to how Zero Two's eyes were just illusions in his angelic form.)

Dark Matter possessed Magolor to try and get close to Kirby to destroy him. But, to beat someone as tough as Kirby, he needed the power of the master crown. He had Kirby and co. do all the dirty work, then he steals the crown in a gambit to try and destroy Kirby once and for all. Were it not for Landia, he would have indeed succeeded. Whether or not Magolor is killed at the end doesn't matter, really, because it was Dark Matter the whole time.

In response to the above, Zero was behind everything.
We know that Zero controls Dark Matter, so it was likely Zero's operation the whole time.
 * Makes you wonder what Zero has been doing the entire time since Crystal Shards...
 * But there's still Kirby Canvas Curse and Kirby Squeak Squad: in Squeak Squad it was Dark Zero, likely a different entity from Zero. And Canvas Curse had Drawcia gain her powers from a deal with Zero. Still, Zero was present, kind of.

Magolor's friend who told him about Kirby is Marx.
It's all pretty consistent.
 * Their
 * Perhaps he is Marx. They both have similarly pointed heads, they, both are/  affable enough, and they're never seen together in the same place. Perhaps Marx went through a bit of a costume change to try his plans again.
 * The only problem with Magolor = Marx is that Marx doesn't have hands, and Magolor doesn't have feet.
 * Look at Magolor's hands. They're not even attached to his body. Is it possible that they are some sort of magic gloves that Marx obtained them after Milky Way Wishes? As for the feet, well, if he's desperate enough for a disguise...
 * Detached hands are hardly uncommon in the Kirby-verse. Examples that come to mind are Daroach of Squeak Squad, or Iron Mom and Jukid of Super Star (Ultra). And even if Marx did chop off his own feet, there's still the body shapes of Marx and Magolor. Marx is all round, while Magolor appears to have a cone-ish body.

The Master Crown is sentient.

 * The Crown isn't only sentient, it's evil.

The Master Crown was created by the same technology that was behind Nova.
This would provide a good explanation for the similarity between the abilities of Marx and Magolor - giant lasers, thorn vines, teleportation, Soul form to name a few. Those spells were signature of the ancients that crafted both crown and comet, and when those artifacts grant power it's those abilities they confer upon their user.
 * This is confirmed in-game, Magolor mentions that the Master Crown was made by the Ancients, and we also know that the Ancients made NOVA as well.

Marx wasn't just Magolor's friend: he was also his mentor of sorts.
It's implied that Magolor's friend is Marx, right? I also think that maybe Magolor taught Marx the same type of spells he uses. In other words, Magolor taught Marx magic. It's why they both have similar magic attacks and. Maybe as they studied magic, they became friends, and started developing plans to take over Pop Star. Don't know why Magolor would wait for so long to do his part of the plan, though. (considering that the original Kirby Super Star came out in the mid-90s, and this game in 2011...)

Magolor is a former Dark Matter.
Like Gooey from Kirby's Dreamland 3, Magolor was a Dark Matter who rebelled against Zero and ran off somewhere, he eventually arrived at Halcandra and lived there for a few years. After Zero 2's defeat in Kirby 64, his soul slipped itself into the Master Crown, possessing Landia and causing it to go on a rampage; at this point Magolor had a few friends and his own ship. After Landia's rampage, Magolor confronted him and lost, causing him and his ship to crash into Popstar.

Now bitter and pissed off, Magolor asked Kirby and his friend to bring back the fallen ship parts, keeping a calm and friendly facade throughout. Once they flew to Halcandra, Landia attacked them, causing the ship to go down. Kirby and friends fought Landia and won, freeing Landia from Zero's influence. Magolor takes the crown for himself and transformed, his Dark Matter side comes back to him allowing him to brainwash the Starcutter Lor into attacking his former allies. When that fails he tries to kill them under Zero's influence, after Magolor is killed, the Master Crown shatters taking Zero's soul with it, freeing Magolor in the process.

Magolor EX is a manifestation of Magolor enhanced with Zero's power, and Magolor Soul is Zero, Lor EX is the Starcutter Lor at full power.

Magolor's consciousness belongs to the Master Crown.
Just... look at Magolor Soul's flavor text! He's described as "a sad shell that is an embodiment of the Master Crown itself".

Landia was possessed by the Master Crown.
That's why its eyes seem to keep changing color, from green to red to blue. At some point before the game proper starts,, it wound up on the dragon's head, transforming Landia into its protector by twisting its desires to suit its own needs. It also forcibly merged the four of them together in order to keep them all under control; their breaking apart upon turning red is a desperation tactic. Blue is their true eye color, signifying that Kirby and the others freed them from the Crown's evil influence.

Magolor wasn't Evil All Along.
He just found Landia being controlled by the Master Crown (related to the above theory) and stole the Lor in a desperate move of defence. Sadly, his goal to defeat Landia turned into an unhealthy obsession with the Master Crown itself, and when he took it off Landia and put it on, it started slowly possessing him until he lost all control and became merely a "sad shell".

The game will be re-released in HD on the Nintendo Switch.
With a new mode "New Challenge Stages" carried over from Kirby's Dream Collection Special Edition to make up for that game not being released in Europe. It will also have an easy way of re-reading pause screen flavour text similar to Kirby and the Forgotten Land.