Some questions on obscure planar lore...

Hall of Heroes for 1st/2nd edition (it's labeled 2nd edition, but most of the stats are 1st edition) mentions the Citadel of White Exile located on the border between the "Plane of Gems" (presumedly, the Quasielemental Plane of Mineral) and the Positive Energy Plane. The Nameless Bard (Finder Wyvernspur) was imprisoned there by the Harpers for centuries. Presumably there's more detail in the novel Azure Bonds or its sequels. It doesn't sound like this is the Tower of Lead, which isn't white, and the Citadel of White Exile doesn't seem to have anything to do with forging - but maybe the Citadel is where the people in the Tower of Lead disappear to.

Dragon #174 says the most common place to gather ioun stones is "from the safety of the great, lead towers that protrude along the border of the Positive Material and Mineral planes."

"These towers, which extend deep into the Positive Material plane, are totally unaffected by the disintegrating power of the energy. Within a few of these bastions live beings of immense power, such as spellcasters of 18th level or higher, powerful creatures from the outer planes, and even powerful visitors from parallel Prime Material planes. From these lead sanctuaries, they perform studies of the Positive Material plane, seeking new ways in which to use that power and thus augment their vast arsenals of magic."

The 1st edition Manual of the Planes says "the origin of these towers is as yet unknown." I'm tempted to propose some kind of anti-Doomguard faction that died out long ago (probably long before the actual Doomguard was founded). If the lumi built the towers, apparently they've abandoned their creations for some reason. Maybe there's some kind of curse they're avoiding. Maybe the Doomguard cursed them!

I feel confident that Teliggin isn't mentioned anywhere else.

Not to threadjack, but the "anti-Doomguard" is basically the same explanation I'm currently leaning towards. Four towers bordering the Negative Energy plane, four towers bordering Positive Energy, it fits the Parallelism theme of the Inner Planes really well. (Not to mention with a PC Doomguard Air Genasi Sorcerer interested in the Quasielemental Plane of Lightning, I know that tower will come up sooner or later.)

It might have been something created by the Wind Dukes of Aaqa to preserve species threatened by the encroaching hordes of the obyriths. Or it might be an experiment by the Ancient Ones, or Risik, detailed in supplement DMGR5, Creative Campaigning. The Risik created a demiplane known as Chanak as a laboratory into which they snatch species and cultures from thousands of different worlds to see how they'll interact (or for other, unknown reasons). The Risik might be using the Paraelemental Plane of Ice to store species they haven't yet introduced to their artificial world. Or maybe Arcolantha was created by a nature god gone mad, whose obsession with preserving life inspired her to freeze every single animal on her world, leaving the planet uninhabited. Maybe the animals aren't "real" animals, but spirit animals that represent the secret part of the soul, and somewhere there's a world where every human is a dazed, hollow thrall, unable to mount any resistance against the tyrant who controls them while their spirit animals remain imprisoned (inspired by The Golden Compass by Philip Pullman). Maybe Arcolantha was created by someone who sought to pull part of the Beastlands into another plane.

The question is what the opposite of the Doomguard would be. They wouldn't merely seek to diminish the flow of Entropy, since there are Doomguard who feel that way. They would scarcely be able to conceive of Entropy at all; the end of things, and how fast it arrives, wouldn't be their concern. Instead, they'd be obsessed with creation, the beginning of things, seeking to research how things began and perhaps create new beginnings. But whoever created the positive energy towers seems to have abandoned them long ago.

Parallelism, of course, has mixed results. The various elementals do seem to parallel one another, but frost salamanders really don't seem to have anything to do with flame salamanders.

I think they should be focused specifically on the origins of the multiverse and the creation of all things. They should be interested in making entire worlds.

The premise of the Expansionists is basically, "all things should grow and evolve," which seems fairly distinct, focusing on progress rather than origins.

I could see the Expansionists as the other faction's heirs, though, continuing the work the original group began. "Okay, we're done. We know where things came from. Where do we go from here?"