A thought: could Maria have been the first "Hunter of Hunters?" The Crowfeather Garb suggests the first came from a foreign land, and I wonder if that implies Cainhurst? Secondly, the Hunter's Hat states that it is fashioned after "one of the old hunters," and whereas most believe that this person was Djura, I can't help but notice that Maria's Hunter Hat looks like a very rudimentary version of the infamous cap. All of the primary hunter hats are pointed to resemble a beak, and the symbol of the Hunter of Hunters--the "Hanged Man"--is highly reminiscent of a crow's talon, after all. Maria certainly doesn't take to another Hunter encroaching upon the secret of Kos, either.
The Crowfeather Garb also suggests that Hunters of Hunters let their victims lie "in hopes they would find rest in a Hunter's Dream." To me, this calls to mind the carcass of Kos, which Maria evidently insists "should be left well alone." Even though Maria appears to have erected a tombstone for Kos and leaves flowers at the grave, she still kept her distance, choosing only to mourn from high on a cliff above the corpse. And just what was this "blasphemous Yharnam burial service?" Apparently it would "trap" the dead in the Hunt, perhaps to be resurfaced some other night. Could this be what those crawling caskets and the One Reborn represent in Yahar'gul?
I also wonder, before Maria became Gehrman's student, what was the Hunter's Dream? It doesn't seem the Plain Doll would have existed yet, and evidently Gehrman was still alive in the waking world (some say he would have been younger, but he was apparently referred to as "old Gehrman" even in the times of Ludwig), so was there even a Hunter's Dream yet? Did the Hunt used to take place in the waking world? The Hunter's Nightmare is unusually bright, despite the Hunt is supposed to take place at night.
When did Gehrman create the Plain Doll? Obviously it had to have been after meeting Maria, since Maria was evidently "unaware of his curious mania," and I imagine seeing a doll that looked just like her would have really freaked her the ***** out. It must have been after Maria's death, perhaps as a form of mourning. Which leads to the next question: when did the Moon Presence take interest in Gehrman? Old Yharnam was the first location to witness the Paleblood Moon, so it seems that the cleansing of the scourge, or possibly just the ensuing violence attracted the Moon Presence. I suppose it was at this time that it noticed Gehrman's worth, unless the Moon Presence was always a part of Gehrman's psyche and was simply awakened during these events.
Gehrman instructs the player to close the curtain on the Hunt, and is desperate to keep them from becoming obsessed, or "blood-drunk," tying in with the philosophy of the Hunters of Hunters. Could this be the influence of his love and grievance for Maria? Could it all be an act of revenge against the Healing Church and Byrgenwerth for what happened to her?
Perhaps one could assume this is what the Moon Presence was intrigued by?
Oops, I deep'd a little harder than I originally meant to.