Just as an exercise, because I've thought about it off and on for years, ...
Questions: Seasons? What's "up there?" and "down below"? What does the source of a river look like? Are there other planes? Is there an underdark? Is there a "backside" to the world disc? Is there really even a "disc" if there's a wraparound effect?
What would your answers be? What other questions need to be answered?
So, way back when, I guess c1985, I did decide on a flat world.
It was contained in a rotating iron sphere, so the 'fixed stars' were actually on the sphere, as were the planets, which moved independently - and literally wandered, not following orbits. What they were - crystals, flames, holes, angels, cities, spiders - was something that was never confirmed.
The sun was a deity (though which deity seemed to depend on who was looking), crossing the sky every day, with schedule & route determining the season. One consequence of that is there is no difference in the length of the day or the season, however far north or south you travel. But quite a difference in the declination of the sun and the warmth received. The moon, OTOH, was a habitable terrestrial land in its own right (which the PCs actually visited at one point).
The in habited land masses were towards the center of the world-disc, surrounded by ocean - presumed, in theory, to be held in by a ring of hills along the world's perimeter, also never confirmed.
The world was thicker than the greatest mountain was tall - plenty of room for an Underdark, though there wasn't anything quite that extensive - but, legend had it that dwarves once did, in fact, dig a mine so deep they fell from the bottom of the world.