The admittedly bat**** theory I've been throwing around in my head is IF the world before the Elvenhan empire was ruled by Dragons, whatever Elgar'nan doing allowing the Elves to rise above them and form the Elven pantheon... What if they Elven pantheon was trying to.. turn themselves into dragons? I know. It's crazy. We can pretend I didn't say it. And again, I have no proof and I'm not married to the idea. I just find it interesting that perhaps they DID succeed - and that's how we end up with the Old God dragons. And while they succeeded, the fallout/ consequences were far worse than they had ever imagined.
Again, I don't actually think that thing and it's just been me trying to figure out why the hell they were stuck in Old God dragon bodies to begin with.
It could also be that the elven pantheon were dragons. At least, anyone from Elgar'nan and Mythal's brood. There's already a fair bit of evidence that there were once super intelligent dragons that ruled Thedas. Presumably they would be able to use magic just like any other creature on Thedas. Humans can turn into crows, bears, ect. It wouldn't be that hard for a dragon to take on mortal form and decide to govern a people. When the bottom falls out on the elven empire, the dragons go off and found Tevinter.
Or, it could simply be that the 'forgotten ones' were another group of dragons that had a similar idea.
Another possibility with the qunari being a mistake- perhaps they weren't intended to be slaves. If the magisters engineered qunari, what if they were perhaps trying to create a hybrid between their gods and the people? Perhaps they are failures because their expectations were far greater than just a race of thugs. They wanted something superior to elves. But what they got was mostly human, prone to violence and unpredictable. The best they could be was slaves. And they weren't even able to control them for long at that.