Please feel free to add your theories. Just do so at the bottom of the page.
Who is "The God of the New World"?
The GOTNW could be anyone in the series, honestly. It's most likely as of Chapter 7 either Mara (due to his hatred of humanity) or the "ancient evil" that the synopsis references. Since it's unnamed and not seen so far, who knows who or what it could be?
With recent revelations, it could most definitely be whoever Mara's master is that he's talking to in secret.
The Man with the Alien Arm is future Mura.
This one is kinda reasonable. He's the main character of the series and it would make sense for him to be disgruntled and broken after whatever happened to him. The alien arm could be something he gets at some point, like an upgrade? Or it could just be a part of his Primal Release.
The Ancient Evil locked away is actually Izanami.
Izanami is mentioned in Chapter 3 as being a goddess who lost the Great Soul War, prompting her to be defeated and locked away. We don't currently know if she's even alive anymore, if she retains her powers or whatnot, but its the most likely choice that's been foreshadowed thus far.