... and I GM a game called Godbound. Coming up with fun challenges for PCs with god-powers is definitely a challenge in and of itself so I'm always looking for ideas and help!
First of all, welcome, Jason!
I do not know yet of specific content for PCs with god-like powers, and the kind of game is definitely different to just high-level PCs, but let's see how we can analyze this. Let's start with high-level PCs:
Recently,
@DM Badger asked about how to challenge high-level PCs in combat
here and there were several answers to it. Combat-wise the challenge can come from heavy decisions like who to save or what to loose, it can also be created by
GM tactics and stratagems. In addition, as I described
here, it can be evolved with situational challenges and successively escalating the situation.
Now, non-combat wise the question is whether the players want to stay on (a) the hack-and-slay adventurer level, or whether they (b) want to evolve their play to more geo-political, world and society shaping gameplay.
In case of the former, it could either be (a1) in the same world with less and less challenges, but more and more display of their might. You wouldn't present e.g. 8 orcs, but instead an ogre army. Those players may love to just cut down one ogre after the other (or several simultaneously). You will want to increase stories about them and the awe they face from peasants. Hirelings might show up and influential people might want to show to the public that they know them and present them with great gifts (magical weapons, exquisite horses or even pegasi, or castles). It becomes a game about fame.
Also, in case of (a2), you could make them travel to a distant realm with high threat potential, like the Abyss or some Netherlands or an Elemental Plane. This way, you can really up combat challenges and they can explore even more (assuming that they already explored their homeworld to such a degree that it became boring for the players). You don't need to stick to standard laws of physics, you can have them travel to flying castles over hovering rocks, you can have them dive into lava pools to find some overlord of the realm. You can clash them with demi-god like entities and make them fight these as in an underdog situation against a demi-god. In this gameplay, you will want to present them with a high level of magic, world-destroying capabilities and maybe even with the challenge to juggle several worlds and their influences on each other. You can have them earn super-powers that are NOT in the book. They will probably LOVE to find their own special powers that
nobody has besides them.
In case of the latter, you would shift the game from physical combat to political and intellectual combat. The physicall prowess of the adventurers needs to be still abundantly clear and a tool for intimidation, but it is the political intrigues and to uncover who are the good guys and who to mistrust that makes this game interesting. Relationships with other cultures and countries play an even more important role in this game.
In either case, you should, no, you must talk to your players about it beforehand to make sure that this is the kind of game they want.
Now, continuing with the demi-gods and gods: I had several games before in which my players either (c) rose to god-like powers or (d) began with these powers.
For example, if they rose to power as in (c) (in our case by defeating a god and inheriting true power), it should be a drastic change from spellcasting and powerful feats to inherent magic. I loved the sessions the most where the PCs learned about their powers and how to use them and how to enhance them and the shock in their faces when they did something over the top they hadn't actually intended. Control is the key word. It is so much power that they constantly need to power-down while knowing that they have abilities that could destroy worlds. But you will also want to present them with situations (after a while) in which they can use them. E.g. an invasion by an extraterrestial HIVE, for which they need to blast all their power into the wormhole they come from. Gameplay will now include other demi-gods. Countries will ask them for help and even build temples in their honor. And yes, maybe they will discover that they become even more powerful, when there are worshippers. They could find out that they receive additional powers BY the worshipping and exactly those powers the worshippers believe they probably have. So, if they want to change what powers they have, they would want to change their image as a god-like entity, e.g. from water and dolphins to water and sea dragons as their imagery. But once they try to change to fire, they will notice that the believe in them crashes altogether and they loose their power. Names become important. Names will echo with them, when worshipped. And they will gradually also get the feeling that these powers, names and the worshipping itself becomes like chains to their existence. They evolve into stereotypes if they want to become more and more powerful. And they will find it hard to break the stereotype. The gameplay shifts to the question of who they are and who they want to be. Belief and Identity could become the pillars of the campaign. But whatever the case, the PCs are still rooted to the world they once inhabited.
On the other hand, if they had those powers to begin with as in (d), then they are entities that are not born. They shaped themselves and shape the world. They need to have some form of ambition that leads them to action. This gameplay is the least adventurous, but the most creational. You let the PCs or rather the players as there is almost no difference here now, create worlds and races, have them combat other gods and their ambitions. You usually cannot die, but you can temporarily not succeed. You will want to influence your own group of heroes to fight for your ambitions, to fight against your nemesis and you want to give them powers and luck. I would create several groups of PCs for this game and maybe let them be played by other players, so that the gods can hear about their deeds and influence their story. I would completely dump the rules system for this. I would want them to be imagining that they could do everything in their sphere of competence. WIth one player that worked out very well. But on the other hand, you could give them trademarks like a power over fire and grade it with 1 to 5 stars to find out how much power they actually have over it in relation to other gods. So, I will not determine specific spells, but rather degrees of powers just like they would do in Mage: the Ascension. I love that system. Combine that with free magic as from the Dark Eye and you have a very powerful system. Now, it should be noted that the gods do not use their powers endlessly. Thus, there should be strain. You could choose to give each PC a number of boxes for their general power and if they use a rank 2 fire power they would need to cross off 2 points of their power, which they only regenerate through worship by others. This sounds like using something like the blood pool from Vampire: the Masquerade. Alternatively, you could opt for a light side / dark side way of gaming as is fantastic in the Star Wars RPG series, where PCs may use their power but then have to flip one of the coins closing them off to use that power too much, while simultaneously giving the option to use their power to the other side, the GM or to another player.
These are just some brainstorming thoughts, but I hope this helps.