So, let’s say, right this second, I invent a new RPG. It’s really limited in scope of what it covers… maybe only “matches in a Magic: the Gathering arena”. The rules? They’re “play Magic decks to resolve combat”.

And… that’s it.

Extremely high complexity / high crunch / whatever, yet almost 100% pure Zeus.

The only thing you might have to and be allowed to make a ruling on are things that don’t come up in normal play, but could here, like (if the rules don’t specify) what counts as an “opponent” when alliances can be made and broken mid-fight (maybe this is already covered in MtG?).

(Ok, as an RPG, arguably you could allow custom content. That would be highly… Hephaestus. People might want to come in with completely crazy things, like summoning troops from Warhammer 40k and ordering an exterminatus or something. )

Note that, if there are no NPCs, you don’t even need rules for “what does it take to convince someone to join your side in a fight” or “will they trade the cards/spells”. If there are NPC characters, I’m not sure whether “roleplay them like they were PCs” is Hephaestus or Atlas.

All that said, the amount of effort the GM needs to put in isn’t exactly a 1-to-1 correlation with the system’s Zeus rating. After all, creating content for 3e D&D is such a chore compared to doing the same for 2e and earlier, for example.