The theosophists believed (okay, made up - sorry, I'm a science guy, so I don't jibe well with the realms of pseudo-history, pseudo-anthropology and pseudo-science) in the concept of root races - races of "humanity" that preceded modern human beings. These races were supposed to be our forebears, and indeed shared the planet with us. In these elder days long forgotten, planet Earth was very different than the Earth we know today, geographically-speaking. The theosophists were working off of some theories popular at the time that have since been abandoned, specifically the notion that continents could sink into and be raised from the ocean depths. Understanding continental drift as we now do, we know that Earth's continents were configured differently back in the day, but we also know that this was not due to continents sinking or rising from the ocean floor.
A Theosophy-inspired campaign might be set in the waning days of Atlantis. That civilization is no longer at its height (this ties into the implied setting in D&D of a medieval world built on the ruins of a more highly advanced world), but it isn't at the point where it's sinking into the sea. We will also assume that the other root races are still knocking around, and that modern humans are now appearing on the scene - that gives us the "multiple races" concept we need for good and proper D&D. Of course, we're going to mess with those races just a bit to make them work better in terms of the game rules.
So, first things first - what are the different races, and where can they be found.
The first race, the eldest race (so we'll call them the elders) were an ethereal race. These folks are actually composed of etheric matter - which in D&D terms means they can only be hit (or attack material creatures) with silver and magic weapons. That doesn't work too well as a playable race. The original elders were formed when Earth was still cooling - their ethereal bodies were unaffected by the heat - but their descendants are less ethereal. They reproduce as amoebas, by dividing their bodies, and are the forebears of the second root race, a golden skinned people. Perhaps these modern elders are halfway between their purely ethereal forebears and their golden skinned descendants. They'll have silvery-grey skin, be slight and graceful, can walk through walls once in a while but are otherwise vulnerable as normal humans are to cold and heat and physical duress. Their slight forms make them quiet (i.e. move silently). Because they reproduce by dividing, they are asexual. While they can be found all over the primordial world, their "home base" is Mount Meru, an ethereal (and invisible) mountain at the center of all creation.
Elders for B&T
Elders are medium-sized creatures with a base land speed of 30. They have darkvision to a range of 60 feet and a knack for moving silently. Since the matter of their bodies is only about 90% material, they suffer a one point penalty to starting strength, but gain a one point bonus to starting dexterity; an elder's starting ability scores cannot be reduced below 3 or increased above 18 due to these modifiers. Once per day, an elder can force their bodies to become completely ethereal, per the ethereal jaunt spell, for 1 full minute. In addition, an elder can reduce its solidity enough to reproduce the effects of the blur spell for a total of 1 minute per day. Elders can multi-class as fighter/thieves, thief/magic-users and thief/clerics.
The second root race are the Kimpurshas (sometimes called Hyperboreans). The Kimpurshas are golden-skinned humanoids who reproduce by budding - so again, no males or females in this race. They hailed from a great northern continent that was tropical, because Earth had not yet received its axial tilt. In time, this continent was shattered and altered, so that the Kimpurshas had to move on. They can still be found in the world as nomadic sea peoples - sometimes merchants, sometimes pirates - moving about in great, ancient galleys.
Kimpurshas for B&T
Kimpurshas are medium-sized creatures with a base land speed of 30. They have darkvision to a range of 60 feet. Possessed of vril (mystic energy) in their veins, they enjoy a one point bonus to their starting charisma scores due to this infusion of energy, but suffer a one point penalty to their starting intelligence scores due to their primitive lifestyle; an elder's starting ability scores cannot be reduced below 3 or increased above 18 due to these modifiers. A kimpursha's magical blood gives them a +3 bonus to save vs. magical effects (spells, powers, etc.) and a +1 bonus to all other saving throws; they are famed for their luck. As kimpurshas are nomads of the waves, they have an innate ability to predict the weather and a knack for keeping their balance. Their climb speed up non-sheer surfaces (i.e. where they have hand-holds) is 15 feet per round. Kimpurshas can multi-class as sorcerer/fighters, sorcerer/thieves and sorcerer/clerics.
The third race are the Shalmalians (sometimes called Lemurians). The Shalmalians are taller and larger than modern humans, and have black skin. They inhabited the continent of Shalmali (Lemuria) in the Indian Ocean and southwest Pacific, a continent that is no longer with us. Their remnant populations now reside in proto-Africa, proto-India and proto-Australia. There were four sub-races of the Shalmalians, the first three reproducing by laying eggs, the fourth as modern humans. I think for our purposes it is more fun to have them be egg-layers (which also gives a nice tie-in to Barsoom!). They aren't as advanced as the Atlanteans, so we'll give them a medieval level of technology and cool weapons and armors based on Africa and India. Since dinosaurs still roam this primordial Earth, we'll also make them the masters of these great beasts, using them as beasts of burdens and as war-beasts. Imagine an 8-ft. tall Shalmalian knight astride a triceratops in mirror armor with lance and curved sword!
Shalmalians for B&T
Shalmalians are medium-sized creatures (though they often grow to almost 9 feet in height) with a base land speed of 30 feet. Their size gives them a two point bonus to starting strength, but they suffer a one point penalty to starting wisdom due to their bravado and tendency to use their strength to solve their problems; an elder's starting ability scores cannot be reduced below 3 or increased above 18 due to these modifiers. Shalmalians have a knack for taming wild animals and using them as mounts. They can multi-class as fighter/clerics, fighter/magic-users and fighter/thieves.
The fourth race are the Atlanteans, of which there were several sub-races. For our purposes, we have the High Atlanteans (essentially based on the American Indians) and the Low Atlanteans (essentially based on the Turanian peoples - a nice tie to Hyborea!). The High Atlanteans built an amazing civilization that harnessed vril (google it!) to power their flying machines, telecommunications, etc. They had aerial battleships that carried 50 to 100 fighting men who launched poison gas bombs and fire-tipped arrows, etc. The High Atlanteans worshipped the Sun as the ancient Egyptians did (their descendants) - so our Lawful clerics are sun worshipers. Unfortunately, they eventually fell into the use of black magic (chaotic clerics and magic-users) and became materialistic - they fall, and the Low Atlanteans (who also use black magic) become dominant. Atlantis is torn by strife and goes post-apocalyptic on us. Some old vril-powered technology (magic items) survive, but most is gone. The people have a medieval-level of technology now and the end is coming soon.
High Atlanteans for B&T
High Atlanteans are medium-sized creatures with a base land speed of 30. Natural magicians, they enjoy a one point bonus to starting intelligence. Their over-reliance on vril-powered technology and fine living imposes a one point penalty on their starting constitution. High Atlanteans are usually well-educated, and have a knack for deciphering codes and activating magic-user scrolls (with a penalty to the roll equal to the level of the spell; one an activation is failed, the scroll is worthless and the Atlantean suffers damage equal to the level of the spell). High Atlanteans have limited telepathic abilities, being able to communicate telepathically with sentient creatures up to 150 feet away. This also allows them to attempt to implant a suggestion, per the spell, once per day in other humanoids. High Atlanteans can multi-class as fighter/magic-users, cleric/magic-users and magic-user/thieves.
Low Atlanteans for B&T
Low Atlanteans are medium-sized creatures with a base land speed of 30. They enjoy a one point bonus to starting intelligence but suffer no constitution penalty, and they have a knack for deciphering codes and can communicate telepathically up to 150 feet, but without the ability to implant suggestions. Low Atlanteans can multi-class, as High Atlanteans, at first level or choose to change classes in mid-play as humans.
The fifth race (and now things get tricky) are the Aryans. Yeah, the people with "moon-colored" skin. They emerge from the Atlantean race and leave Atlantis to found other kingdoms - specifically on an island in the shallow sea that will become the Sahara Desert - The City of the Sun, and on a similar island in what will become the Gobi Desert - The City of the Bridge, located directly beneath the etheric city of Shamballa. These people are guided by the Lord of the World and will eventually become many different peoples in the modern world - Hindus, Arabs, Persians, Greeks, Celts, Teutons and Slavs. To play this campaign, you'll need to get over the old idea of the Aryans being a master race, and just play them as "normal humans". If necessary, drop the word Aryan.
Aryans for B&T - as humans in B&T
So we have a large campaign map and several races. Classes are according to whatever version of the game you play. The point is still one of exploration (primarily in the ruins of Atlantis) and treasure hunting. Lawful clerics want to reestablish Sun worship, so maybe they want to enter Atlantis in search of holy relics. Magic-users are searching for ancient Atlantean wisdom - perhaps all magic in this campaign is powered by vril, and the magic-users are trying to discover the secrets of that science, now lost to mankind (but still lurking in the hollow world with the