It would be interesting to see that since, if I recall correctly, the Qunari have never had to weather a blight. What would an invasion of a demonic, almost unending horde do to them and how would they cope?
But then again, I would like to see a Qunari invasion in it's full might, no longer stymied by having their Arishok cut off from the other rulers (I am afraid that their titles escape me at this moment).
I suspect that a Qunari Invasion would have to start with the removal of the Imperium and the capture of Tevinter, otherwise it would have an enemy at it's flank and that hurt them badly during the last war.
But the problem with that is, the fall of the Imperium would likely be witnessed across Thedas, Antiva, Nevarra and even Orlais trades with them.
There really can be no surprise for Thedas, and that is something that would be critical for a successful Qunari Invasion, But the problem we face here is we do not know the number of ships the Qunari possess, nor their number of troops.
I couldn't even tell you if they have the logistical basis or manpower to conquer Thedas.
They failed utterly last time, in no small part due to just the sheer numeric superiority of Thedosians.