On Flat Surfaces
As we all know, it was once assumed the world was flat. Yes eventually some ancient scientist figured otherwise, but just imagine if the world really was as flat as a scroll. Now imagine a world as such in your game, where the pesky laws of gravity and ph
As we all know, it was once assumed the world was flat. Yes eventually some ancient scientist figured otherwise, but just imagine if the world really was as flat as a scroll. Now imagine a world as such in your game, where the pesky laws of gravity and physics need not apply. Much wondrous flavor and imagery can be brought to a new setting using this dynamic.
The Oddities
Since we have broken the laws of physics, making a flat world, we need some way to replace the necessary ones. For instance, gravity. To justify a world being flat, gravity can't  be functioning as normal, otherwise it would round out. We can fix this problem a number of ways, one might even be scientifically plausible.  As a first, let's look at magic. Using magic we an either decide to dismiss the problem all together, or we can come up with some interesting history. For instance, perhaps your creation myth includes everything being bonded to the earth, or maybe great wizard built towers that hold everything below their height down, because no man can exceed their glory. Now for the fuzzy science explanation, the one hinted at earlier. If we can assume mass brings gravity, why not put an enormous object underneath our planar world, or extend the world itself downward infinitely. One most notable examples of the formal would be Terry Pratchett's Discworld series.1
Years ago people looked up to the stars and said, "Hey, those organized shapes, they must mean something." So they made stuff up. However these constellations would cycle, and eventual became a largely used way to tell the season, without taking into account everything else. On any world that might rotate in someway, this aspect of time management becomes lost. The answer is simple, make it move in some way. If it infinitely extends downward, as per earlier example, have it rotate on its center axis. Yes, the constellations won't come and go, but it this case it is position that matters. Just move it in some organised repetitive way and you can justify any pattern of constellations you may want to introduce into your world.
Water flows, and on a flat world sometimes it flows off. Where does it goes? Does it come back? How? Let's answer this with a question. If a druid can create water, what can a god do? I say a god may be well in his right to create a inexhaustible supply of the element we plenty for. In other cases a lake centered on our world might be the closest thing to a sea, and run off is unheard of. Another possible world would be one headed for ruin, as the entire climate is becoming dry, and desolate. Perhaps the god who had created the aforementioned endless supply of water has ditched, and the heroes need to bring him back.
Messing with the Players
On a flat world, large scale cartography becomes plenty more simple. The mapmaker need not world about sill projections as there is only one possible way to show it. Likewise sailors can look to a star and say, that is north, without worrying about magneto-polar navigational techniques. All of these are in game changes, that players would probably not notice. Let me tell you what though, with a flat world there are a myriad of ways you can mess with your players.
With every flat world come the world's edge, now you just need to get your player there, along with someone to push them off the edge. This might take a fast magical method of travel, or it might take the trek across all of the known lands. Either way, you are sure to get a response if you put the party between the edge and an army of red dragons.
"The other side of the world." Think about that. Now think about a coin. Now think about reversals. Now think about what would happen if you place your PCs in a world that is completely opposite to their own and they changed sides every time they went to sleep. Now imagine if someone decided to connect the two sides of the world. Here you have the seed for a campaign if you have the time, and wish, to cultivate it.
I like to call the next idea the crumbling world. It is pretty self explanatory, but I can see this going extremely well if a GM had set dates, and measurements of how the world would fall apart overtime. I can see this going a number of different directions. One bei ng, the PCs save the world and prolong the corrosion, and in oppositions, the PCs standing stacked, foot on should, balancing on a speck the size of a penny which would soon vanish.
Conclusion
Flat worlds provide many an attribute to augment a setting. Most of the inconsistencies can be solve, either by work of magic or fuzzy math. In addition there are many things a GM can do to slowly dribble away at a players sanity, using flat worlds. If you have ever used flat worlds, or worlds in any other interesting shape, in your game I would love to hear about it. Join me next time when I build a world out of a tetrahedron.
1- In which case we have a giant turtle flying through space