The Inquisitor. The Inquisitor takes his/her crew to the insular Dwarven city of Kal-Sharok. From there they mount a deep roads expedition with the purpose of investigating red lyrium, Primeval Thaigs, and the Blight. The Warden who is researching the Calling is also in Kal-Sharok because all the Dwarves there are rumored to be immune to the Taint.
Throw in some Dwarven politics. Add some side stuff with Varric/Bianca and Morrigan/Leliana (if the Warden makes the cut) as playable companions. Make 3-5 new areas including the city of Kal-Sharok as a hub. Throw in some Carta side quests. We've got a decent 15-20 hour expansion on our hands.
The plot is okay I suppose, just don't like the idea of the Inquisition investigating Kal-Sharok, when the headquarter of the Grey Warden Order is right there, close to Kal-Sharok. Also, don't like it when the taint/darkspawns is getting treat more and more like...well, not dangerous that normal people can fight without getting injured/tainted. I also don't want to go to the Anderfels/Weisshaupt/Kal-Sharok in a DLC. The Anderfels is too much of a clean slate that I feel that it will work better as DA 4, or DA 5, not a DLC.
If we are getting a Warden DLC, I rather play as the Warden, similar to Leliana's Song where we play as Leliana. The main reason would be the personality of the Warden, it's easier give a sense of personality to our Warden if we play as him/her, rather than as an NPC or companion. Hawke has a set personaility and people still complain about it in DAI, I can't imagine the backlash from the Warden.
I saw a post recently saying that Leliana's agents tracked Solas and found that he was last seen heading West. The Warden is in a place far West looking for the cure to the Calling. I would love it if we play as the Warden and meet Solas, not knowing that he's the Dread Wolf. Maybe Solas has something in mind though or he shouldn't be there in the first place...