Many thanks to Kevrj for responding! Here’s something I just wrote about XoR for a friend that may add further detail of interest on the “inspiration” question:
Game and lore design for XoR 1
I started designing it in 2011, just after I read Choice of the Dragon and decided I needed to write my own CoG. Back in the late '90s and early '00s I had run a lengthy D&D campaign around the theme of a slave revolt against an evil fantasy empire, and decided to take the broad plot arc of Rebels from that game. But I carried over almost none of that gameworld’s lore; I didn’t want to use its orientalist tropes, its elves-dwarves-and-dragons Tolkien borrowings, its D&D-esque version of blood magic, or its nine cosmic threats held back by nine talismans (too many MacGuffins, and a distraction from the more interesting political themes). So the Rebels gameworld was written starting more or less from scratch.
I didn’t try to write all the lore from the outset in 2011! The game concept gave me my starting point: an evil empire that harvests the blood of its slave caste to fuel its tech/magic. The empire justifies this by pointing to the threats along its borders. I drew the basic world map with that in mind, also knowing that most people would be reading this game on a mobile, not able to see the map – so let’s just have one province in each of the cardinal directions, and a major threat also roughly corresponding to those directions, as that might be easier for readers to mentally keep track of.
Because the story started in the periphery of one province, I also knew I didn’t have to come up with everything all at once, but would have some chance to “discover” the world as I wrote it, along with the reader/main character.
The first advice I’d give anyone in game writing is to read nonfiction on topics you enjoy thinking about, and weave the real-world dynamics that you find interesting into your game – that can help you escape the trap of just repeating features from other fictional universes. I had decided the empire would draw heavily on Greek culture, so when I started deciding how the magic system would work, I thought of what I knew about Aristotle’s science and the concept of telos. I did a little more reading and research to help me develop a fantasy system with a tinge (not a lot!) of real-world philosophy. And as I’ve read history, philosophy, and politics books over the past decade, I’ve always kept one eye on what might fit well as part of the gameworld. That has helped me put more and more flesh on the bones of the original concept.
As for how progress is going: I’m afraid I have not yet finished my first draft of Chapter One of Game Two. COVID-19 has hit my productivity pretty hard this year; as I’ve said a couple other places, my day job at present is leading a $10m a year charity in Nepal that among other things runs two hospitals. As you can imagine, that job got a lot more complicated this year, with fever clinics and isolation wards and the need to fundraise to fill a gap created by the collapse of patient income during Nepal’s nationwide lockdown!
But my successor as director may be chosen earlier than expected – this week, potentially! – and that might allow me to hand over earlier than expected and switch to a lighter workload from mid-2021. That would open up more time for writing. And when my current term as a missionary is done, in 2022, I’m planning to write full-time until the Rebels series is done.
If Zvad is alive at the end of Book One, you can see him again from Book Three onwards.
Yes. Horion talks about this in Chapter Three, if you indulge him down the right conversational path.
You can find information on the wiki. Happy to answer any specific questions that brings to mind!
Yes! I’m going to do my best to make sure I don’t add any fundamental stats that might require me to update XoR 1 stats and void all old saves. 
And thanks for your kind words about the game; and I also love how active this thread is.
@Bluebirb, I’m speechless. Thanks so much for sharing such wonderful encouragement.
I’m afraid I have a policy of not helping with mods, amigo – not because I have any objection to players modding XoR, but because it takes me enough time to figure out how to make my code do what I want it to do, let alone what other people might want it to do! Best of luck.