I have been getting promptings to add to the Atlantis myth for a long time. That is why I keep coming back to this subject. Atlantis is one of my passions, obviously. I've been watching a series on Magical Egypt, to see if I can get any ideas about what life was like in the mythic past.
At the moment, I'm adding a couple of special materials to D&D/Pathfinder. Titanium Steel and Transparent Aluminum (which is a technology I think only we have but I can be wrong about that). Transparent Aluminum (Aluminum Oxynitrate) is a special material that is, well . . . TRANSPARENT! :) It is also virtually indestructible, and is capable of being molded into glass like material. I don't know if it can be sharpened, but for armor I do believe it will work because of its lightweight properties. Just that modest conscious people will have to wear underthings.
I have been mulling over doing a 4e version. I don't know about that, though. I still don't understand the SRD perfectly. The OGL version is the one that is being written right now. Atlantis is a place of high technology, magic, and psionics.
This particular episode of Arthur C. Clarke's Mysterious World shows what I'm talking about. In Atlantis, you will be able to play psions, psychic warriors (or battle minds), wizards, sorcerers and other classes. This is the ancient mythic world, and there will be things available not usually available in a fantasy world. Like submarines, SCUBA, and genetic engineering. When a city of survivors is forced to live under the sea, you have to make certain advancements to live under the sea. Domed cities would not be impossible without certain technologies -- and transparent aluminum is one of them.
I've been getting promptings for a long, long while. And it's about time to get it all done. I have what people said about Atlantis. Most everything really, including Plato's letters to Critias and Timaeus. I think I can draw on everything said about it. Instead of being a terrestial city (which there are many RPG books written on that subject), I'm going to do the underwater aspect.
Beneath the waves of the Sea of your campaign world are three domed cities . . . the Kingdom of Atlantis.