I think when it comes to the slavery issue that Dorian might have to make some tough decisions and decide what his end goals are. Really, the type of slavery that Tevinter practices is not sustainable. They need to have a continual supply from nations all across Thedas just to keep it going, which we saw elements of in DAO and DA2. The Imperium would fall apart without its slaves, but Dorian doesn't want it to "fall apart," he wants to fix it. On the other hand, sometimes it does take drastic change for there to be ANY change, so perhaps some sort of slave revolt is the only thing that will drive home the point to the altus in power just how precarious their situation is.
As much as I admire Dorian for his effort in this, it just seems futile to me, particularly in the hints that we got in Trespasser of how he's going to be doing it: joining with Maevaris in the formation of a new political party of fellow magisters that want real, positive change. We know how Alexius was before the whole mess in DAI, and we also know how Dorian's father was. So the entire lucerni party is not going to be composed of idealistic youths.
Even though I thought the difference in letters between a friend and romanced Inquisitor were nice, you do get more insight into the political process from the friend version:
Inquisitor,
It was good to hear from you, my friend. For months I've had only the Society of Maevaris's fledgling Lucerni party: Junior members of the Magisterium so filled with fire and zeal, and so wildly inept at politics, Mae keeps a bucket of ice water on hand in case one accidentally immolates himself. Lest I give you the wrong impression, we are making progress. But it will take a great deal of skill to keep the Lucerni alive through the usual schedule of Minrathous scheming long enough to become a real political faction. Fortunately, they do have me.
I'm sorry to hear that politics are plaguing you as well. Must be something going around, like a pestilence or an Orlesian fashioned trend. Hopefully Josephine can defuse the Fereldan outcry and persuade the Orlesians to stop circling you with a collar and leash. You know she did always love a challenge.
I'll find an excuse to make a trip south soon. We should really catch up in person, don't you agree?
Dorian
At the end of Trespasser we learn that the Qunari renew with vigor their campaigns against Tevinter. Obviously we don't know the plot of DA4, or what the significance of the post-credit scene (knife through Tevinter) will have for the next game, but I can say one thing for certain: if there is a large-scale slave revolt while the Imperial Army is trying to put down these Qunari incursions the Imperium WILL fall. Their slaves support the army, and they will need an army to put down a slave revolt as well as to continue in their efforts against the Qunari. They will NOT be able to manage both.
We also learn from the Dorian/Bull romance slides that the Venatori are still a thing and still a threat in their own right, so that's something to be concerned with as well.
Dorian knows all these things. He is not naive. I think at some point he will have to admit to himself that what Tevinter really needs is to be rebuilt from the ground up, by people who care more about her and her people than about themselves and their own power. It will take time, but I think he can certainly be the one to do it.