I've wondered about something - it was my running theory before everything in Trespasser came to light, actually - and it's got a few holes now but I might as well put it down.
I wonder if people and spirits aren't binary things, but two halves of the same whole. Like, people act as though they're separate entities - people are spirits who became people, and you can only be one or the other. But let me propose a slight tweak to that theory - people people, the people Solas talks about, whole and healthy - are spirits and a physical form. That everyone has a "spirit," and everyone has a "body," and before the Veil, mostly everyone's spirit and body was perfectly in sync with each other. It's how Solas can be both a spirit, but also "grow up" - because he was never only a spirit. He always had a body, and that body grew just as any other does, but he also had a soul - a spirit of Pride, or Wisdom, or whatever. That we can't think of Solas as "just a spirit" or even "a spirit that's taken physical form," because it implies that a spirit is "whole," and the body's maybe just an add-on. But what if that's not the case, and spirits nowadays are just as shattered as people are?
"But Ella!" You're probably not screeching at your computer screen because that'd be weird. "We know spirits are fine. We've talked to plenty of them. Oh, sure, you got your nasty old demons, but the non-corrupted ones seem pretty alright." Are they, though? We know that a lot of spirits seem to be lacking a sense of identity, individuality - they name themselves after things, "Compassion," "Command," "Wisdom," but don't really take names for themselves, right? And Solas says that only spirits that have a strong sense of self can survive in the physical world - almost as though spirits that don't are lacking something. And what's odd, is that what they lack is the exact opposite of what people nowadays lack. People lack magic, mutability - they're too real, too physical. Spirits though, spirits lack permanence, lack stability - they are not real enough. It follows, then, that they way to fix them is to combine them. People - Cassandra, Josephine, Vivienne - are just... shards. But so are spirits - Wisdom, Faith, Command. They're severed halves of a single whole, lacking what the other has too much of.
Which means that Solas is that perfect whole - a body completely in sync with it's spirit. It also means that Solas is a perfect abomination - a physical being that has completely entwined with it's metaphysical "partner." It's because of this that so many demons seek to possess something (Which, remember, they remain in the Fade as they do so - recall back to Origins, where we had to go into the Fade to kill the demon possessing Conner.) - they want to be whole with their physical half. And it's why so many abominations end up going crazy - because it's like trying to cram together two puzzle pieces that don't fit. Even "good" abominations like Wynne and Faith still think of themselves as separate beings. They might be a better match than most abominations, but they still aren't just right for each other - or maybe they are, and the Veil keeps them from fully intertwining.
People as they currently are sort on a sliding scale of connection with their other halves. Dwarves have no connection, maybe because they don't even have another half (Spirits have little to no interest in them, remember). Tranquil have a completely severed connection. Most have a very, very weak connection, which is why they don't really dream of the Fade or attract demons. Mages have a slightly stronger connection, which is why they do dream of the Fade, because they are more in touch with their spirit there (Which is probably how they exist in the Fade to begin with - meaning that, when a mage dreams, they "slip in" to their spirit self and muck around in the Fade that way. This might also explain how Solas knows Lavellan's spirit even though she's apart from it.) And Dreamers, rare as they are, are so in sync with their spirit that they can effect the Fade as any other spirit would - and act like a beacon to all the demons seeking out their other half because of it.
But none of them as in sync as as Solas, as any ancient elf, which is why he needs to tear down the Veil, so they'll no longer be severed from their own souls.
It explains a lot of other things too - the way Solas says Lavellan has a marvelous spirit, despite having a body and being a physical thing - because she does, she's just not completely in touch with it. It's why he seems to use "soul" and "spirit" interchangeably, despite them being very separate things in Dragon Age. It explains that dev comment on how there are "different opinions" on Solas being possessed (Which you'd think would be a pretty straight forward call.) It's why he says Sera is "apart from herself" - which wouldn't make much sense if she were a spirit given physical form, because her entire "self" would be right there. Why he says he grew up, even though he should've been born fully grown if he were just a spirit. Why Cole says he is in both places, even though (Like Sera) he should be all "right there." And it explains why Solas must tear down the Veil to restore his people - not so spirits can come here without worry. He wants to do it so everyone can reconnect with themselves.
It might even explain Justinia in the Fade - she's both a spirit of Faith and the Late Divine, because Justinia's soul was a spirit of Faith.
There are some issues with the theory, though - for instance, Trespasser contains a reference to "brothers of the air," that haven't taken physical form - which wouldn't make sense if they have to have physical form by necessity. And there are spirits that have lived for thousands of years, despite bodies obviously not living so long (Although, perhaps they are the souls of people who have died already). And of course, there's Cole. Don't know where to begin with Cole.
Eh, just figured I'd throw it out there anyway.
Also sorry if a lot of this was already brought up. Shows me right for spending, like, two hours trying to gather my thoughts on how to say this.