Ok, please stick with me for this. If a someone who wasn't a scion, changeling, had wizard's constitution, or marked by power; picked up sponsored magic somehow, would it be a possible for changes to occur to their body because of the alien energies flowing through them? The character in question has been gifted with Summer Magic but is a Knight of the Cross. People who become Fae Knights have to take Marked by Power which I could see as having a secondary trapping of filtering the energies associated with the sponsor so that they don't completely screw up the mortal vessel. If someone didn't have that to refine the energies or a better than human constitution to take care of the damages, could they begin to change physically in a way that would befit the sponsor? Seelie/Unseelie would become slightly fae like in appearance, and depending upon whether they took any powers could possibly be mistaken for a changeling. Locations could cause the person to look more in line with the "feel" of the location (volcanos could appear to have embers in their eyes, skin could become ashen, etc.), Hellfire wielders might begin to look demonic, and Soulfire users could appear to be angelic.
What are your thoughts on this? Mechanically I could see this as a way to justify buying creature powers and possibly at a very advanced stage inhuman-supernatural stats. Fluff-wise, it could be a very interesting story to see this character change the more they use their power, and further separating them from humanity (actually, you could buy off debt with mutations-non power mutations that is, you don't get anything for going into debt).