er.. if you don't mind some seriously tin-foil theorizing, and start with the assumption that all spirits and demons are essentially souls (some of which manage to maintain a sense of self or memory after their physical death) it's possible that the Forbidden Ones are the souls of some of the more powerful rebel leaders killed in the failed Revolution. Since they actually died, they weren't put on trial and sentenced to void imprisonment like the Forgotten Ones, but Falon'Din considered them too much of a risk to reincarnate.
Solas: "Some of your forces, valuable until now, have no interests beyond creating disruption. Chaos for its own sake. They must be repositioned where they can do no harm, or removed if necessary. You replace them with organizers willing to build a new system and carry out the ugly work that must be done."
The bolded part is suggestive, at the very least. They threat they pose as demons is less than they would if reborn, and thus they remain trapped: disembodied and angry spirits with full knowledge of blood magic and an interest in sowing chaos.
This is all supposition, of course! But it's a plausible explanation to consider.
sorry madrar I forgot to reply to this because of CK2
OTL
Anyway, if I'm getting this right, what you're saying is the first demons were punished dead-so-already-in-the-Fade elves, probably lieutenants of Fen'Harel/Falon'din's first rebellion?
...I don't know *holds head*. I'm still trying to wrap my head around your theory. My brain always goes "heck no" whenever I read it, though I'm trying to give it a chance and my rejection's no longer quite so violent lately. I'll get back to you after I've managed to wrap my mind around it some more.