The game world is based upon the Wheel of Time and the concept of Mirror
Worlds.  The Wheel of Time is essentially the "true" world, the universe
closest to the True Source.  The largest part of what this effects is
the magical systems of the other worlds, which consists of almost any
published d20 system.  (Though the more diverse the campaign settings,
the more difficult pulling the pieces together are, particularly for
spellcasters among the players.)

The other constants within the setting are few, but they exist in 
basically the same form in every setting.  The Dark One (a loosely 
imprisoned over-deity of evil), the Creator (the Dark One's good counterpart), 
Tel'aran'rhiod (the World of Dreams), and the Plane of Shadow (basically 
the void in which the Mirror Worlds float, though access to it/the planes 
can be restricted) are really the only constants between the worlds.


Magical Systems

  Wheel of Time
    Channeling works directly in the Wheel of Time, the One Power is the
    source of all magic throughout the multiverse.  Saidin is tainted as
    usual and the Dragon has just been born, the bonds upon the Dark One's
    prison beginning to fray.  Channelers of either type of energy, saidin
    or saidar, have access to a range of abilities unseen further from the
    True Source, which, among other facts scholars quote, is one of the
    reasons that it is believed the Westlands and Seanchan are closer to
    the true source than any other plane of existence.

    If a caster from a plane farther out on the Wheel of Time visits this
    "base" world and can use both saidin and saidar, being this close to the
    world polarizes the character.  The player chooses one or the other form
    of channeling and from that point on can only use that form of magic/the
    One Power.  Characters arriving in the Westlands/Seanchan for the first
    time from further out along the spokes of the Wheel of Time generally
    develop special abilities of some sort.

  Rokugan
    Divine and arcane magic are analogous to the saidar and saidin respectively,
    but the taint, due to being channelled through the elements which control
    magic in that setting has diffused through both.  While spellcasting can be
    accomplished while avoiding the taint, succumbing to it is swift and is not
    necessarily restricted to spellcasters.

    The Shadowlands are an equivalent to the Blight in the Wheel of Time, but,
    due to the filtering effect of the elements, they serve as a magnet for the
    taint and are thus that much more virulent.

    Travel to Rokugan among the Mirror Worlds is largely restricted due to the
    restrictions upon the Void/Shadow element in that setting.  There are only a
    few Portal Stones that reach the Mirror World, most directly from the Wheel
    of Time.  (Current Campaign:  The artifacts which bind the Inner Planes to
    mitigate the effect of the taint upon Rokugan were nearly thrown out of
    balance after being mistaken for Portal Stones.  While the elements nearly
    absorbed the Prime Material Plane, they were eventually brought back into
    alignment.  The plane of Shadow is now open and Rokugan is more easily
    traveled to.)

  Forgotten Realms
    Divine and arcane magic are analagous to saidar and saidin respectively, but
    the Weave filters the taint out as completely as possible.  Ao within the
    Forgotten Realms is a neutral counterpart to the Creator and the Dark One;
    he largely takes no part, but did step in long enough to establish the
    Weave before tasking its maintenance to Mystra.  Shar's Shadow Weave draws
    upon the True Power and her desire to see the world destroyed is largely to
    allow it to be rewritten in Shai'tan's image.  The gods within the Forgotten
    Realms have complete control of access to both saidar and saidin.  (Shar is
    actually the counterpart of Ishamael.)

    The great glacier hides the Blight.

    The Forgotten Realms are a nexus among the Mirror Worlds, one of the stronger
    realities from which others radiate (Rokugan and Greyhawk are among these)
    similar to the way all realities emanate from the True Source.

  d20 Modern
    Due to its distance from the True Source, magic is all but unknown within the
    d20 Modern setting.  Like the Forgotten Realms, d20 Modern is a nexus among the
    far flung Mirror Worlds and for those Mirror Worlds that emulate it in some
    form but are closer to the True Source, magic does exist in some form.  In
    those Alternate Material Planes where magic cannot normally be accessed, the
    use of artifacts or technology will be necessary to use any supernatural
    abilities.

    The d20 Modern setting is currently in the year 941, and most of the gods upon
    that plane have died off due to the drift from the True Source and a lack of
    belief amongst mankind in their divinity.

  Dragonlance
    Saidin and Saidar are each split into two forms of magic, arcane and divine
    resulting in four separate forms of magic in Ansalon.  Saidin split into the
    two "natural" forms of magic:  primal sorcery and mysticism.  Saidar is split
    into the god granted or formulaic forms of magic:  high sorcery and clerical/
    druidic magic.  Due to the distance from the True Source, the taint is not as
    pronounced upon saidin in Dragonlance, but it is beginning to be felt by
    practitioners of primal sorcery and mysticism now that the world is once again
    rejoined to the Pattern of the Ages.

    Set just after the War of Souls; the gods are desperately trying to curb the
    use of sorcery and mysticism though their followers are unsure of just why
    they wish to do this.  Rumors of erratic behavior by practitioners of natural
    magic are beginning to spread, however, in the years since Takhisis was
    killed.  The gods are forbidden to intervene directly, however, due to the 
    conditions set down by the High God, a neutral counterpart to the Dark One 
    and the Creator much like Ao.

    (When under the influence of the taint, a sorcerer or mystic can overchannel
    using the rules presented in the Wheel of Time book.  Also, once the world is
    returned to the pattern, a choice between natural and formulaic magic is not
    a possibility.  As in the Wheel of Time, a caster uses either saidin or saidar
    though it is not necessarily based on gender.)

  Dark Sun
    As with many of the areas further out from the True Source, saidin and saidar
    are analogous to arcane and divine magic respectively.  After the Cataclysm
    the use of arcane magic fell out of use entirely due to the ability to choose
    which one could use, unlike in the Wheel of Time.  However, eventually defiler
    magic was discovered, in which one uses the life force of the planet as a sponge
    for the taint, mostly cleansing saidin (albeit not completely, as evidenced by
    the undead t'liz).

    Set in the time period described by the Dragon Magazine update of the Dark Sun
    world.  One alteration to that, however--all Dark Sun characters are gestalt.

  Star Wars
    Saidin and saidar are akin to shallow pools of stagnant water in the Star Wars
    campaign setting.  Saidin manifests as the Dark Side, while saidar is seen in
    the existence of the Light Side of the Force.

    The campaign is set on the world of the Sith, just after the renegade Jedi have
    arrived and been declared gods by the Sith peoples.

  Eberron
    Haven't given too much thought to the saidin/saidar split yet, but in this 
    particular version the Quori are likely to play a large part due to the presence 
    of Tel'aran'rhiod.

    I haven't read Eberron as closely as I could, but that seems like it could at 
    least be a starting point to begin thinking about this.  For Eberron, Syberis/
    Khyber certainly seem to be the "high god" equivalents to the Creator and Dark 
    One in the Wheel of Time.  Their shards could be the focal points through which 
    the One Power flows into this particular Mirror World.

    The existence of Eberron dragonshards presents a slight problem with this 
    picture though, or at least an imbalance.  Three dragonshards and only saidar 
    and saidin to split between them--I think the easiest route might actually be to 
    leave it as it is in the Wheel of Time.  Men use saidin when casting, whether 
    they're using divine or arcane magic, and women use saidar.

    It wouldn't really be possible to have the taint common due to the prevalence of 
    magical ability in Eberron, unless perhaps certain spellcasters learned to reach 
    through the dragonshards to enhance their abilities/power.  That could make for 
    an interesting custom prestige class actually (exclusive to men/saidin, and 
    perhaps allowing a small number of arcane spells for divine casters and vice versa).