I feel much the same way as Mutton on the flat-Galbar idea.

The Mk. 3 universe’s theme is Aristotelian physics, which was quite clear about the world being spherical itself (you always hear about how the Greeks figured out that the world was round thousands of years ago, but not about how strange their model was with everything else being arranged in spheres) so it seems both thematic and appropriate to have a round world. 

I don’t see much purpose to a flat world since it’s going to bring the aforementioned weirdness with horizons, break the concentric spheres theme, and not really offer any advantage beyond some measure of uniqueness.

[quote=@Double Capybara]
I feel like trying to set up the world without knowing our gods will be hard, especially now that planes have such a functional role...
[/quote]

Right. Much of Mk. 3’s world will depend upon the gods that we get. A sun, ocean, and some islands are guaranteed but that’s about it.