Dorian's personal quest should've involved actually going to Tevinter. When you get the quest you might not even realize it is his personal quest. Just you and some diplomats, maybe even Josie 'n Cullen, going to meet with some potential allies. They don't even have to go to a major city or anything, they could meet up with the contact at an "undisclosed location near the border" (some noble's summer house). The background designers for the game loved the Frostbite engine (unlike everyone else) and found it relatively easy to make the maps, so I don't think it would've been too much of a stretch to do.
But before u even leave, Mthr Giselle pulls u aside and tells u of the letter from Dorians father she received, how she doesn’t know what’s going on between them, but he seemed like a sad old man who missed his son and hoped u, dorians friend, could make sure they meet and maybe reconcile. (Also don’t tell Dorian he might not go on the expedition shhh)
Heck, the contact could even be Maevis! the cool translady in the comics and the DLC. She sends her own servants/slaves. Maybe she sends a boat to pick you up, there's a cutaway and we're already at the manor. It would give a good opportunity to talk to her, the slaves, and just get different insight from other folks from Tevinter than just Dorian--who definitely has things shrouded by his personal priviledges.
But once you go inside the manor, that’s where the confrontation happens
-
You learn more about their situation, like how his father kidnapped him from alexius's house and kept him under house arrest. Maybe it would play out in the vanilla game where you need to make a decision right that moment, or maybe everyone puts on a veneer of "everything is fine. let's just smile and do business". A good representation of what Dorian lived with and was expected to do his entire life, a chance to see the manipulation and abuse firsthand, in a subtle manner.
Or maybe the whole thing was a trap from the beginning. that his father is so desperate for his son back, to keep his son safe from the Inquisition that he's willing to kill for him --Or it could be one of many options! Maybe it only plays out this way if u don't encourage dorian to give his dad a chance
But if you do fight, I don't think Dorian would kill his father. He still loves him, would be heartbroken that despite everything, his father still doesn't understand why Dorian left. So he just leaves him instead.
if YOU kill him instead, or go behind his back to do it, or don't convince Dorian it's the right thing to do, you WILL lose his loyalty.)
I dunno, just a thought.