If you're using Ascii, then commas or periods are a great way to show chasms. I think that it encourages safe and slow play, since you have to check all around you when you move, in case a chasm were to be around. Maybe have a "Chasm/Trap Detecting" skill that would increase the radius of which you could see the chasms. A lot of other roguelikes I've played have something like this, such as Dungeons of Dredmor. However, I've always found it annoying when a roguelike had traps in a large area, since I'd usually want to quickly run across the area instead of taking things slow. Another way I could imagine doing this would to make so that commas or periods didn't appear, and the only way to be able to tell if there were traps/chasms would be to throw an item and trigger the said trap/chasm. Of course it would be annoying throwing items wherever you walked, so maybe have the chasms only be in certain areas or rooms, with a sign leading up to the area waying "WARNING! CHASMS AHEAD". But again, if you're looking for entire dungeons with randomly placed traps/chasms, I'd just go with the commas or periods for those closeby.
