Because I'm a pedant: ancient people did *not* think the earth was flat. Anybody who has seen the horizon know that it's round, unless they really believe that the world ends approximately twenty one miles from wherever they happen to be standing.

I'm not a fan of non-standard world shapes: I don't see how it will make the blindest bit of difference to any of the inhabitants. It strikes me as a rather pointless gimmick.

About the only exception I can think of is what you might call a "mythic shaped world", where North, South, East, and West are more symbolic directions than anything else: like the Elemental Poles in Exalted.