Tevinter is watered by Fenris' tears...
In all seriousness, I wonder what the relationship between the Tevinter Warden Commander and Imperium leadership is?
I would assume it to be a cordial, working relationship, given that Tevinter has suffered the most from the Blights historically (shattered the original empire, made them weak enough for some barbarian woman to sack their lands, put all their Gods to sleep, millions dead from both the Blights and losses to rival powers over the centuries, etc.) and they would have a certain appreciation for Wardens - at least, fellow Tevinter nationals. In my mind's eye I can see the Tevinter Warden Compound in Minrathous - vast halls filled with mosaics and marble statues that move only sometimes, vaults stuffed with lore and Joining elixirs both failed and potent, an endless supply of slaves for research and testing, laboratories that make Avernus' look like the backwater post it is.
Logically, they should have accomplished much, much more than Avernus has. Blood magic is pretty much an open secret in the Imperium, practiced behind closed doors, and no door is more pointlessly closed and secret than the Wardens'.
Then again, given Bioware's track record, I expect the Tevinter W-C to at the Archon's throat for no reason at all, if they even make an appearance. There just might be a single crude chemistry stand, tucked away behind a "warrior-only" bashable door.