So, since I can't mess with the direction of gravity yet I thought I might try cutting a sphere in 6 segments and arranaging them so that characters on one of them won't be able to see the others thus providing the illusion of being on a sphere).
The actual sphere will be bigger but I kept this small to able to get it in one shot (this one hase a radius of 100 blocks).
I'm think perhaps a radius of 500 to 1000 blocks will make the map large enough to be interesting but small enough to still notice the you're on a sphere.

I thought I could put walls around the edges of each segment and organise portals to allow travel to segments on the other side of the wall.

Top of the world.

Heading to the wall.
Rather than trying to generate landscapes I'll copy sections from the regular random maps and overlay them on the sphere (why bother recreating that when the existing mapgen works so well).