I'm imagining some kind of catastrophe that happens. hundreds of thousands or millions will die. Then a Powerful arch-mage opens up a pocket dimension and the people escape into it. This mage can control every aspect of the demesne: weather, looks, features. Everything. He makes a perfect habitat for the people he's saved.
Fast-forward 1000 years later. The players live in this place. The wizard is immortal (somehow...a lich? Wish? Strange Pacts?) and is worshipped as a God King.
What kinds of cool adventures can I plan? What do I need to build into the world?
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I kind of like the idea of the group doing adventures in the 'world' but slowly finding out that things are not as they seem. That there are cracks on the edges. Then they find out there's 'another world' and, possibly, that their God-King isn't a god at all.
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Does anyone want to take a stab to help create something?
Sure! This is a great setup.
I think it would be most interesting as a failed paradise built by a genuinely compassionate archmage--worn down by time, weariness, and the accretion of good intentions gone bad--rather than as his (i'm gonna call him 'he' because 'king', but a she would work as well or better) deliberately self-aggrandizing dystopia.
My responses in no particular order are:
what is the rising conflict? Is the mage going mad and the world is starting to crumble? Is the catastrophe from the outside making its way in?
The mage controls every aspect of the domain, but he is not a proper god. He couldn't over 1000 years maintain a paradise and society that nourishes the aspirations of all the refugees he saved. And, as the society has grown less fair, in spite of and because of his interventions, he has grown jaded and apathetic--no longer exerting the effort he once did to keep everything in the paradise safe and functional. This has created a vicious cycle: apathy -> system failures -> more apathy -> more system failures and, since the archmage controls every aspect of the domain, his neglect is a massive problem. By the PCs time, things have gotten dire.
a whole religion revolving around the God-King. What does this religion look like? Do clerics of this god get powers?
2. The wizard didn't intend to become a god, right? He wanted to save folks and then find a way out. So it'd be counter-intuitive that he'd use this as an opportunity to seize power. Instead, it's possible a religion sprang up, no matter what he wanted (aka Dune) and he is behind the scenes seeking to quash it.
So, if that's the case, he wouldn't be empowering clerics or even warlocks within the religious hierarchy that deifies him. Maybe they don't have any magical powers... or maybe they're getting their cleric powers from somewhere else knowingly or unknowingly.
a cult that believes King is a false god. There is a better place where greater gods exist.
The inciting disaster wouldn't destroy the gods and leave one dude's pocket dimension standing. They'd still be out there and might still respond to people who actively worship them. The god-king's false religion would suppress this, of course.
what do actual Clerics look like? Are other religions banned?
Maybe they look real weird--if none of the old world's faiths survived and contemporary people had to construct new belief systems from whole cloth, or in opposition to the god king's worshipers, they'd be heterodox iconoclasts.
What kinds of laws are there? bans on certain kinds of magic? (we don't want anyone accidently dispelling the dimension or traveling outside and finding out the truth)
So, my thinking is that this is an issue that might not come up. Being a deliberately created paradise, there aren't really spell scrolls and forgotten lore sitting around in dungeons anywhere or monsters that inhabitants could fight for fun and level advancement. From the start, the archmage would inherently control most access to wizard magic. Maybe sorcerers possessing pocket dimension destabilizing magic by accident of birth would be considered a problem.
You'd want to work out what class levels mean in the setting and how they are gained, whether spell knowledge arises independently of class advancement, and, if so, how.
an elite force of Knights dedicated to protecting the God-King
Horizon Walker rangers--or at least that general idea. The archmage trained them to keep the pocket dimension safe from the catastrophe and other planar weirdness. They're insular free agents entirely separated from the priesthood of the god-king.
There is a place that is considered unholy that no-one must enter (The exit to the demesne...) What is this place? What does it look like?
The unholy place is the part of the pocket dimension where the strain on the constructed paradise is most extreme. It is a place where the systems have broken down: crazy weather, nonsense physics, non-euclidean geometry, monstrous creatures, and literal holes in the dimension that can be leapt out of. Obviously, this mythic wilderness would be a good place for the PCs to spend a fair amount of time.
I think the unholy place would be coolest if there was a geographic barrier separating it from the inhabited parts of the dimension. A clear barrier, but not something that's particularly hard to cross--a river, not a mountain range--but which it is strongly taboo to cross.
What does crime and trade between cities look like in a land that always has perfect weather and great crops?
This is probably the most important question for setting the tone and feel of the setting.
Is it a paradise of comfort and safety that takes your premise to logical but odd conclusions, i.e. a
froopyland? There wouldn't be much need for specialized crafts or exchange in that case.
Or is it just a high functioning late medieval farming economy with lots of surplus? In that case, the tradeable goods wouldn't be too different: mostly textiles, with rural surplus being exchanged for high value-added crafts in the urban hubs.
The first option would be more fun to invent but maybe a little to wierd, and might give away the twist.
What was the catastrophe? Was it a natural disaster? Invading Aliens? Nuclear War? Disease? Zombie Apocalypse? What does this place look like now?
3. Catastrophe. Mystery and imagination may be the best way to play this. Something bad happened, but no one is quite sure what. Preserves an aura.
Don't decide now. When it comes to the point in your campaign where the PCs have a chance to visit the old world, the best way to go would be to base it on what happened in the campaign up to that point. If the players speculate about it, steal one of their ideas--that way they get to feel clever and included in the setting lore.
