Army needs reform. Slaves will run with the moment of opportunity. Citizen soldiers fight but they are still not good enough. The Imperium needs a strong professional army, just like Rome. It will lead to a new era.
From what I understood, Tevinter already has a professional army (that slaves are not allowed to serve in), but it obviously needs to be reworked. I'm thinking that one of the biggest problems is that, uh, many of our mages do not seem to understand and comply with the rigors and lifestyle of military service. I know of multiple counts of "magisters" traveling to Seheron with slaves in tow, as if it were some holiday vacation instead of an active warzone. Clearly we need stricter guidelines for magisters who wish to participate in combat that act in accordance with military procedure.
Out-of-character: I admit I have not read Stolen Throne, but so far everything related to war and military bodies in Dragon Age makes about as much sense as a soup-sandwich.