Well, I am mostly basing this idea on what we have seen so far. If it's indeed a person or a mage's "sanctuary", it departs considerably from what we have seen so far - mage sanctuaries reflects the user in a way, and tend to be rather positive.
The two glimpses of the dream place was 1) an empty church painted in cool and dark shades; 2) featuring broken glasswork, suggesting abandonment.
So either it's an old piece of the dreaming, or it's from someone who still remembers christianity - and for that to be the case they had to be either alive at the time of illness, or they have to belong to a group that still believes in it.
Either way, whoever it is behind this chunk of dream, I think they will be an antagonist - and that indeed, we may very well get a mage fight.