I like where you're going with a lot of this - and it's certainly up to date with the research I've been doing myself. I think your point about the importance of the Primals vs. Ascians. vs. Woken match is right on, and maybe even opens the door for what we see. The ascian in the Limsa Lominsa storyline wanted the key personally and is visibly a cause of the star showers and possibly even time travel. It would be very interesting if the ascians were the cause of our echo - even indirectly, as if seeing the threat of the ascians roused a primal to pick us as their defense against them.
The goblins are most definitely a unique race - but if we see their primal, I'm betting (like the moogles) it will be something semi-canon; a fun distraction. Goblins are supposed to be ambiguous - we never see their true faces (though, if you do a little subtraction theory on their various masks, you come to something that looks a bit a specific version of the XI concept art, though less scraggly), sometimes they're friends, sometimes they're foes... For them to summon a powerful, legitimate primal would give them too much of a polarized identity.
The same with the qiqirn, methinks. The Aht Urhgan expansion was basically a middle eastern doppelganger of everything we already knew, and the qiqirn were the goblins of that doubling.
Therefore, it makes sense that, even though they'd need a main element simply for the fact we have to fight some of them and they need to have drops, that they wouldn't take up one of the key slots on the wheel.
The sylph, however, are new. In past Final Fantasies, sylph were very similar to pixies - but here we see them with a new visage (possibly to leave the door open for "XI-Legitimate" pixies later on). They have no ties to ambiguity, are part of the main story, and can fall from grace without messing up how we look at the world.
I've got this tingling in the back of my brain that says something very similar to this... Garleans... secret primal... possibly Alexander... I like where you're going with this. [MOOSE APPROVED STAMP]
That's an interesting way to look at it - humans sure don't all look the same, after all. I mean, none of our historically bottle-necked species have grown tails or horns yet (sadly), but there are sure variations in skin, features, and overall fuzziness (lolmiqo'te). I think you're onto something - one species, many different isolated adaptations.
There are many ways to take the former sentence, but I like the theory of the latter. I'm going to add that one to the list. The voice-over you hear during the crystal scene sounds a lot like the voice during the awakening, as well. Good theory, sir!