The Beast Planet is Earth in an alternate Mortal Engines where the plan came to fruition.After London destroyed all the other cities, either with the Wave Motion Gun or by eating them, it eventually covered the entire world. Then it dug into the Earth's core to harness the heat. With powerful engines, it devoured the entire Solar System, ending with the sun itself; then it went on to become the terror of the galaxy...
Bone's political instability is because it is the most independent of the planets.They have organic technology (which lessens a need for minerals), huge stockpiles of explosives (which may indicate a source of energy), food, and likely water to grow said food. Because they don't necessarily need to import or steal nearly as much, their ruling class doesn't need to have the unified front that the other societies need as to not get wiped out.
The Beast Planet is Unicron.First up, the manner in which the Beast Planet eats planets Tek, Fire, Jungle, and Reptizard is highly reminiscent of the way Unicron at Lithone in Transformers: The Movie. Also, the planet Botanica explored to get her plant mode looked very like Planet Jungle; while this may simply have been a simple case of recycling an environment to save on costs, it's much more fun to think the two series take place in the same universe. And if Shadow Raiders and Beast Machines exist in the same universe, that means Unicron must be out there somewhere. Also, Unicron seems to have some new power every time it appears, so Beast Planet's ability to create drones isn't a strike against this theory. A fun theroy is that the Beast Planet is what became of Unicron after his failed attempt at resurrection in the G1/Beast Wars timeline. His body was revived as a soulless machine who continues to consume planets but no longer has Unicron's consciousness driving it.
The Cluster have met interstellar populations before.Cryos in the first episode mentions that the Beast Planet is a myth, a boogyman to frighten small children. How the hell would they have heard about it if they didn't have some contact with someone who had some contact who had etc. etc.
The Beast and the Ancient Race are the sameAt some point or another, the Ancients who created the world engines and teleporters became the Beast Planet. At some point in the distant past they transformed their world into a single massive weapon of war, which, given their world engines, would be child's play since the Alliance already did that to their own planets in season two. The Beast/Ancients just took it to the next level. They built it up bigger, armored it, armed it, and now they're on a rampage. Or, alternative, they created the Beast as a kind of weapon—perhaps they were fighting, and even losing, an interstellar war and were forced to create such a weapon to defend themselves...and it didn't work. Like at all. In that case the Ancients may have actually engineered the warlike Cluster and their neighbors on other planets specifically to fight the Beast, like a failsafe.
The Beast Planet is a Reaper.Black with red lights? Basically invincible? Completely unstoppable, boasting weapons more powerful than any other species'? Stick a few tentacles on it and we've got a Reaper.