Okay, cleaned up and with updates… Warning: Long post.
I see arguments for Aberration, Fey, and Outsider, and Unique:
Outsider: I’m not a huge fan of this one. Athas has no gods or outer planes currently, and arcane magic turning someone into an actual divine being in the absence of deities feels off. I could get behind the idea that the magic tries very hard to do so however.
Under this interpretation Avangions would have to be some sort of arcane angels or phoenixes, but the fundamentally different source of their power, taken from the land and their own complex nature, which would make them feel very different.
That said, I do see that the whole disappearing thing at stage 10 and aspects of their aura just scream outsider. Clearly, the intention was angelic. But it is also clear that there was not nearly as much thought put into Avangions back in 2e as there was for dragons, which has resulted in them being very hard to parse (they lack some common outsider abilities and resistances, even if they do have a few common angelic traits in their aura and detecting evil and lies).
Aberration: This one is a bit of a personal opinion but I think it might make some sense. An Avangion could be seen a kind of a magical and psionic super organism that is a living wellspring of life energy, and is perhaps related to Rajaats research into life-shaping, as could dragons.
As an otherworldly feeling entity, created by the unnatural but connected to nature setup of arcane magic on Athas, aberration can actually make sense. Unnatural is the key word. Avangions are no closer to being natural creatures truly native to the land, or any land, than Borys or Rajaat.
Fey: Pyreen are fey in 3e Legends of Athas, so there is precedent for land connected creatures being fey in Dark Sun, and 4e went even further with making sand brides fey, for instance. While Avangions aren’t technically druids or otherwise bound directly to the land, their powers are connected to the health of the land and its restoration. Fey could work for them, in terms of becoming otherworldly yet land connected forces better than outsider does. It can give them a divine flavor without truly needing divinity.
Furthermore, I will note that the 4e fluff possibly supports them being fey, though weirdly neither epic destiny (dragon or avangion) actually changed the characters type (or origin, as I think it was called in 4e).
Unique: I see why some people want this, given how difficult it is to come to a consensus. My objection is that no other advanced beings have a type of their own, and I think that if that route is taken then Athasian Dragons, being different as they are from dragons of other worlds, should also have a unique type. The issue is that in 2e Athasian Dragons were considered dragons for the purposes of a few magic items and effects, so the dragon type has a historical weight for them (also in 4e Borys had the dragon subtype).
Dragon?: As for dragon type, I think it’s a little absurd to make Avangions into dragons. There is literally no support of any kind for such an interpretation, except maybe some hearsay attributed to Troy Denning 25+ years ago that discussed his original thoughts on Nok and dragons in Dark Sun.