Not trying to dismiss the question, but I struggle to understand the interest for non-flat worlds. If you think about it from a "realistic" perspective, DF worlds are not big enough to form spheres (or at least not big enough to form spheres and also have earth-like gravity). Although, the linked article asks the question, "Can torroidal worlds exist", I will spoil it by saying that they can't form naturally -- accretion disks have a maximum particle size. Above that particle size, the particles come together with their own gravity and form moons -- that eventually clear the disk (see the rings of Saturn for a great example). And even if you built a torroidal world, the size you would require in order for it to have earth-like gravity would be much greater than DF allows (though, I have to check the density of slade to be sure...)
To me flat maps make the most sense -- you are playing on a section of a spheroidal world. The map is a projection onto a 2 dimensional square. It sucks that you can't go beyond the edge of the map but that's a technical limitation of the game. Not only does it make the most sense, but it is also exactly what you find with most fantasy literature (probably because everyone is copying Tolkien, but still...). I remember thinking that even Naruto has this square map and no mention is ever made about what happens when you wander off the map (despite being able to go to the moon...).
What might be cool is to have some explanation why you can't get through these square sections -- and/or possibly making the sections non-square. For example, in a site, your other-worldly influence can only extend so far and when dwarfs are outside of the site, they are no longer influenced by/visible to you. Similarly, Armok's view only extends to a section of a planet and so the god seals it off so as to avoid unwanted interference. But I am rapidly descending into suggestion territory.