So I'm gonna sidestep the protracted back and forth about gods, what they are, what the dark/doom-sun cannon is regarding them, etc., and reply to some cool thoughts from earlier in the thread.
I suspect that if WotC makes 5e DS, PC templars simply won't be a supported option.
But if I was doing it:
Background: Templarate
Skill proficiencies: Investigate, Intimidate
Tool proficiencies : something or other goes here
Feature: you are granted a Templarate Medallion by your patron sorcerer-monarch
Magic Item: Templarate Medallion
This medallion is personalised to you, and forges a connection between you and your patron sorcerer-monarch. It acts as both an arcane focus and a holy symbol for you. Your sorcerer-monarch can contact you telepathically at will, can cast Scrying on you at will with no saving throw allowed, and can use your senses at will as per the Beast Sense spell with unlimited range and no requirement that you be willing. You can also telepathically contact your sorcerer-monarch at will, though it is rarely a good thing to come to the attention of your King, and you'd better have a GOOD reason. In addition, you learn spells from the following table according to your sorcerer-monarch. You can cast these spells once each per day, and if you have the Spellcasting or Pact Magic features you can also cast them with spell slots. These spells always count as spells known and spells prepared for you. Wearing this medallion openly marks you as a servant of your sorcerer-monarch, which may draw strong reactions from others. [table of spells goes here]
Sorcerer-kings can personally empower warlocks, clerics (of appropriate domains), and paladins (with the tenets of the oaths rewritten) to serve them, but they can also have psionicists and fighters etc in their service. It just makes the templarate more interesting, flexible, and varied. One thing I'd require though, is that in DS, power comes from somewhere. Any divine spellcasting class (a class who can use a holy symbol as a focus) must explicitly either choose a sorcerer-king to follow, or must pact with the elemental spirits.
I immediately liked your writeup while reading it but, upon reflection, I think it would be better to go a slightly different way with templars.
My reasoning is that it isn't fun, as a player, to be a micromanaged by a megalomaniacal boss, even for someone who sees appeal in working from the inside of a corrupt system (for the perks, to do good from within, whatev). That leads pretty straightforwardly to your first point, that templars don't make sense as a supported PC option. On the other hand, the relationship of templar to sorcerer-king might work more comfortably for players if it were relatively hands-off; they are semi-autonomous actors, like the archetypal police detective. Oddly the example that comes most readily to mind is K from
Blade Runner 2049, with the tortuous baseline tests (i.e. wierd loyalty tests), and the understanding that if he leaves his job he will be immediately killed.
PCs could approach and occasionally cross the line of what their sorcerer king deems acceptable as long as they (appear to) get the job done.
Functionally, I'd keep everything from the rules you just described, except that the sorcerer king can't cast scrying or beast sense or use telepathy through it (nor could the PC). Instead, the PC can't take the Templarate Medallion off and the sorcerer king, upon learning of a PC's betrayal or dereliction can detonate the medallion, which--instead of being a bomb in the skull trope instant death--casts a defiling spell that destroys all water the templar and companions have, leaves a telltale defiling residue on him/her, and paints a beacon on the PC that other templars can follow--and shortly will.
That gives templar PCs the latitude to be part of a normal adventuring party, with a continuous tension around their degree of loyalty to a corrupt system.
I think...
I am going to say that the Dragon-Kings went to the Moons and are trying to take them over to reestablish their sovereignties. "Moons of the Dragon-Kings" has a great vibe to it. [...]
Yes. Get what they can from Doomspace and get the heck out of there. They make their Moon Kingdoms really just out of a need to slate their extreme narcissism before they leave.
With refugees from Fyreen escaping partially into the multiverse, word spreads of the Moons of the Dragon-Kings. Fear spreads with it. These near-almighty beings, if allowed to escape Doomspace, could cause a great catastrophe any material plane they go. And what if one entered Celestia and defiled the gods? What kind of power would that bring about? Or if one invaded Avernus and defiled the fighters of the Bloodwar?
The Dragon-Kings cannot be let out of Doomspace. No actually good god will send their solars into it. The Astral Elves have tried, and failed. Out of ambition, mind-flayers and gith have tried, and failed. Still, the Dragon-Kings cannot be let out of Doomspace. If one ever reached Eberron, or Ravnica, or Theros, worlds full of bounty and life, and just started defiling it, it'd kickstart a new genocide and a bloody war. Doesn't even matter if the Dragon-King loses, because the casualties would be immense.
Yeah, a cool vibe indeed.
...which made me want to riff on it.
So, taking your premise, dragons-kings gone to the moons as a temporary stopover before fleeing Doomspace to defile other realms. What if you leaned into Dark Sun's scarcity themes there? Maybe, in addition to simple narcissism, none of the dragon-kings have enough juice to leave--haven't defiled enough stuff to maximize their mystic power, lack a big enough fleet to break past the solars guarding the exits, have collected insufficient McGuffins, etc.--and are in a zero-sum game, racing against each other to get enough of it from the moons before the moons, Fyreen, et al. spiral into the Eye of Doom. For some reason the Dragon Kings are unable or (...because Dark Sun)
unwilling to work together. In the interim, they are making the (assumed) slow death of Fyreen into a fast death, using up everything it's got to fight each other and build ostentatious lifeboats that they will not share.
Also, it might be fun--in addition to seeing what the PCs do and how it impacts the dragon-king's plans--to contrive some way to dice out which factions win in their various conflicts throughout doomspace, so that you get to be surprised along with the players which and how many dragon-kings get eliminated in the struggle.