I truly hope the Tevinter has harsh guidelines for mages.
Yearly Harrowings starting at a young age - contests of power amongst peers - invasive "scientific" tests to gauge ability - invasive "scientific" tests to study magical aptitude... basically grinding mages through years of harsh living to separate the best from the rest.
Mages who don't survive should be dissected - whole families should be wiped out if "defects" are found (like a line that shows excessive possession).
And I hope they take an even harder line on the "Magic is meant to serve man, and never to rule over him." Because magic would just be the means to their power - a tool for them to grasp and control - not to be controlled by. Being controlled by magic (like possession) should be the most abhorrent thing in Tevinter because of the weakness your will showed.
I assume Templars are privately owned? I hope so. I'd love to see Templars used in House vs. House politicking with cruel, resourceful Templars having a chance to be important parts of a family (maybe even honored with joining by marriage).
I'd also like to see useful slaves "rewarded" with breeding - even if they don't get to keep the child or have any contact. After all - there are many traits a Tevinter magister might want to add to their gene pool to increase their magic (I'm looking at you elves). Who knows what secrets breeding elven stock into your line could help with. Though - of course, one must be discerning about such things (and potentially secretive).
Tevinter fascinates me for all the reasons I would find it reprehensible... I hope Bioware doesn't let me down.
Edit: Dorian left because he's gay and refused to breed I believe.