Long prior seeing this thread, I'd wondered about a wrap-around option to the world (relevent to Adventure mode, mostly, although in Worldgen it would mean tiling of the world-map without discontinuity. But of course an 'Asteroids'-style wrapping, that'd produce a topologically toroid world. (But without non-Eucilidean distortions to the plain, no real indication as to which are the major and minor radii.) I'd also wondering, though, about worldgen being given the option of, as well as wrapping in the above sense, also in a spherical one.
For a spherical implementation, however, the 'true' surface of the world would become heavily distorted towards the poles (might be considered more detailed, on the planar world-view, for any given width[1], rather than making it unity) and the pole itself either 'just beyond' the top/bottom edges or represented by the smallest largest relevent circumpolar line's terrain detail within the respective pole's 'row'.
It would need the smoothing, pathing, area handling routines of worldgen altering to make non-grid (more vectorised, in practice) decisions as to topological smoothing/roughening (or fudge by considering extremely polar areas to be glacial/pack-ice with very little variation anyway), but when it came to embarkation, the projection of a square fortress area projection would come out increasingly trapezoidal (to the "world grid) as you reached the extremities, if not completely askew, when actually encompassing either of the poles. On the whole, of course, and assuming that the 'fortress' (or viewable by Adventurer) area is not a significant proportion of the sphere as well. (But then you have to consider viewable horizons, and how far the 'offing' (
Meaning 1, extended to land as well) is, anyway.)
But, to be honest, I've already taken this sort of thing away into the realms of a browser-based game of my own (only on local machine, so far, because I no longer maintain the hosted service I would normally have used for this, for all the necessary back-end scripting needed to represent the view as the <TABLE>-based rendering (as a precursor to dynamically creating true images) that show both map and viewpoint detail). And this isn't DF-based (though I suppose I have thought about adding 'mining for resources' to the equation) and has recursively PRNG-sourced vector-data at the core of its terrains to allow minimal storage for an arbitrary LOD (though I might need a 'diffs' storage if I ever make the landscape dynamic/alterable). So it's not so much something I'm suggesting for DF, but am explaining as a consideration I've been making on that idea of my own. Hopefully it's sufficiently distinct.
[1] The choice of spherical projection would indicate the whether the height gained or lost resolution/detail at the same time.