To a certain degree there already are Primordials in Planescape canon, and it's not a 4e thing. While 4e has grouped all of the elemental princes (like Imix) and powers (Akadi, Grumbar, Kossuth and Istishia) into Primordials, the concept of Primordials is something that's quite common in Myths. Obviously Greek Mythology being the most apparent source of such beings with the Titans whom the Gods overthrew, and who certainly do exist in Planescape canon. And Norse Mythology has the Jotun, who are in some ways also Primordials. There's Pan Gu and Nüwa from Chinese Mythology, the Asuras of Hindu mythology can sort of be seen as such beings. And Tiamat as described in Babylonian mythology (and not D&D where she's totally different) was also such a being.
Though 4e's concept of Primordials, seems to me to be one they ripped off from White Wolf's Exalted, which I never even played or know much about. But the backstory from Exalted seems to be where they tried to invoke something from the concepts of such ancient beings and actually succeeded in having a compelling myth origin. 4e certainly falls flat of what White Wolf did with Exalted.
However the way Primordials are generally presented is in a much too simplistic way, usually with Primordial=Bad. While most of the Greek Titans were seen as bad even if they it was only because Chronus opposed Zeus, Gaia (who sided with the Gods in Exalted backstory, 4e has no such equivalent) and Rhea are usually seen as the more benevolent Primordial powers. Nüwa the Chinese serpent Goddess could probably be in the same category as Gaia and Rhea, being sort of the Earth Mother types. While it's certain that many myths go on about forces of chaos which the Primordials represent who carelessly destroy and recreate. But there are some like Pan Gu and Ymir weren't quite forces of destruction, but rather ones who's death created a lot of the worlds.
Going into Paizo's latter articles of Dragon, they even went far enough to suggest that Queen Morwel and by extension the Eladrin Court of Stars were in fact Primordials. And I know there's plenty of references around Planescape and other D&D canon about obscure ancient powerful entities.
So putting aside the specifics of all these myths, what do you think the role and real dark of the Primordial Powers are?
While I think some of them are certainly forces of nature, many of them are just like any other Power out there, except they've been around longer.
But I'm interested in what some other thoughts are, and what might there be that could be a Primordial Power.
A lot of the obyrith backstory from Fiendish Codex I ended up as primordial backstory in 4e, with primodials becoming the oldest demon lords. Even more so, the demons vs. Gods backstory in Fiendish Codex II pretty clearly evolved directly into the 4e primordial story. Apart from that, the distinguishing quality of a primordial is its elemental nature - the gods are Astral beings, and primordials are their elemental counterparts. Ymir, born from ice and mist, would certainly be a primordial, but recasting the Greek titans, other than Gaia and Rhea, perhaps, would force some changes. Not all 4e primordials are evil, though most opposed the gods. Many of the gods are evil, of course. I tend to assume the four elemental rulers must have either sided with the gods or made some kind of truce.