Are you sure the Other Side is all there is? The way you describe it it seems like everyone who dies ends up there, but I always thought of it as an abandoned place "between nowhere and forever", "the place you go when you have no place to go". Given how many times Hell is referenced as a different place (probably), and knowing that the Other Side is canonically Limbo, it seems to me that only certain types of people would end up there. The Soul Rangers are a clear canon example of this: Their deeds in life were so unspeakable that nobody wanted to deal with them, so they ended up there via paperwork. The Harbingers and other Legends were too notorious (and terrible) to move on after death, so they remain tethered to existence, in one way or another, as long as they're remembered. Maybe everyone who dies in a nightmare or as a result of a terrible murder ends up here too. So on so florth.This would also help make a bit more sense of the numbers assigned to each character, since they should be in the sextillions for even the oldest of them.
"The Game" is a brilliant concept, but as a whole, it strikes me as a bit forced. It doesn't account for the expansion games either, and its timeline contradicts the one you already established when talking about the keys. Keys! The Gatekeeper calls them "My keys" in the original game and at least once more, so wouldn't it make more sense to think the Harbingers are harnessing his power instead of recovering their own? It'd also be a better fit for the Black Key (You dare steal my stuff? Fine, I'll curse it!) and for the change in mechanics in TSR (The Soul Rangers got all the keys during Harbingers, used the power to banish The Gatekeeper and now that he's in the Black Hole they're affecting them negatively).
Structurally, I think you can lose the game descriptions and rules. It's already a long read and they're essentially paraphrasing already existing material. The book as a whole could easily triplicate in size once it's properly redrafted and formatted, with all the pictures and stuff added in. But hey, that's just my opinion and I am no editor.
I really look forward to discussing some of these either here, in the subreddit or somewhere else. All in all, a great diamond in the rough I definitely want to see more of.
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