I've taken one of Cullen's epilogues and created a new area in-game based on what little we know of the sanctuary he's opened up.
The sanctuary is named Wellsprings. Being that the Maker is alternatively called "The Wellspring of All" in the canon lore, and Cullen is pious, it seemed fitting.
Moreover, I've decided that the entirety of the land that Divine Victoria was said to have given him was the Lake Luthias area in general - found in the Hinterlands (days north from his hometown of Honnleath). This land is essentially a "knight's fee" and, while the term is not canon to DA, those were lands different from Freeholds that were given to knights in medieval times. Works for our game's narrative, either way.
Politically, this area would have fallen under the Redcliffe Arling's jurisdiction. So, we'll assume this land was chosen by the new Divine, in part, as a slight against Arl Teagan, since he wasn't a big supporter of the Inquisition. Effectively the Chantry muscling in and annexing the land to give to one of their own albeit the fact that Cullen is from Honnleath and a war hero puts Teagan in an awkward position to even contest the decision.
Anyone wishing to role-play a templar character seeking to kick their lyrium habit is welcome at Wellsprings.
I may implement a minor plot here, as well, but at a later date (we're already starting with 10 plots as it is). I was thinking, perhaps, some apostates in the area occasionally cause a bit of trouble because they don't want the recovering templars there. We'll see.
Cullen is still an adoptable character either way. He'll be NPCd until he's taken, if needed, though.