Cosmo is a Disaster Master in disguise.Specifically, he'll be the one encountered in the Space Kingdom, due to the star pattern on his coat. It's entirely possible that he used his mad scientist skills to create some sort of cloaking device and is pretending to be a citizen. Furthermore, he appears immediately after the scene in which the Nightmare Knight is resurrected (and seven beams of light shoot from the castle, most likely releasing the Disaster Masters). Presumably, he's the most capable of the Knight's servants, and is out to foil the heroes through use of Obfuscating Stupidity - his inventions have always 'malfunctioned' in ways that could have killed them if they weren't lucky, and always while they were attempting to travel to another kingdom.
The Nightmare Knight and the Oracle are siblingsJust a hunch.
“Queen” Cordelia is the Space Kingdom princess.We know there's a Space Kingdom. It may not be on the map – at least, not the one in Saturday's tower – but where else do spaceships come from? Queen Cymbal even mentioned it once. Cordelia constantly hints that she's from Space Kingdom. She rants about what she'll do to this planet, how she'll take over this planet, how this planet shall bow down to her, etc. etc. She has those “weird ears” similar to COMMANDER CABOODLE, CHAMPION OF JUSTICE! (Who, interestingly enough, seems to know about her evil scheming. The main characters wouldn't have found out if not for Lord Cabbage and the Dream Oracle...) Her color scheme also fits the disaster stone that bears the power symbol. Whether she's a princess or not is just a guess, but this troper thinks it would add to her character a great deal. Plus, when Nightmare Knight inevitably betrays her, she and Peridot can (begrudgingly) join the heroes' side.
Saturday is the Flower Kingdom princess.Her ears have sprouts on them, indicating she's from the Flower Kingdom. The first book says that the Flower Kingdom is known to some as "a land of dangerous ruins for intrepid adventurers to explore!" Such a place might inspire an adventurous princess. And besides, what kind of thief has a butler? One that's a runaway princess with a royal servant. Also, Cucumber's reaction would be funny, which is a good enough reason for this series.
Noisemaster and Legato are Co-Dragons, conspiring to overthrow the Nightmare Knight.It's been theorized that Legato is working for or in conjunction with Noisemaster for several reasons: two of the tentative Disaster Stone designs seem to match them perfectly; what looks like Legato's speech bubble color shows up in Noisemaster's broadcast from Rhythm Ridge; Noisemaster talks about scheming with an unseen partner-in-crime; and legato, or "tied together", refers to smooth transitions between notes in music. It's possible that these two could actually have ambitions of their own. Consider that...
Also, the Noiseblaster can be powered up with the screams of the princesses (or just ordinary people, too).Think the Scream Extractor from Monsters, Inc., and we know Princess Piano is pretty much the loudest character in the comic so far. It's also why Legato is so gleeful over finding Nautilus, because it means they can power it up even faster now. (Second part Jossed for now, but might still come back.)
Noisemaster and Legato are somehow related to the king on the other side of Intermezzo Wall.Lute referred to Legato as a sorcerer and said that he was at least a century old (we don't know how long the wall has been up, but the Melody Kingdom does have rumors about it being full of ghosts on the other side), so he could've been the king's magician. If Noisemaster didn't have anything to do with the aforementioned rumors by just wreaking havoc over there, he might actually be the other king; he made some sort of deal for power with the Nightmare Knight to get revenge on the rest of the Melody Kingdom.
Cucumber will break the cycle of the Legendary Hero once and for all.Most of the other characters have been stealing the spotlight, but eventually his character arc will end in a lesson about how there are multiple ways to be a hero (or that nobody HAS to follow the genre 'rules') when his extensive magical studies allow him to stop the Nightmare Knight and Disaster Masters from being brought back every 5,000 years. The Nightmare Knight might even be redeemed.
Legato is some kind of ghost.Not a whole lot to say, but he does have strange powers, still looks young despite being "an ancient evil", and he might have some kind of unfinished business or mental block keeping him from passing on to the other side.