I have been trying to structure this for a while, and I am not sure how to present it so it makes sense, so I will structure it via what I consider the "hook," the way all of the elements tie together. This is, of course, wild, random speculation of the worst kind. 
I. The First Magic - Stars and Stones.Why is Mortal Magic so important in regards to summoning outsiders? Because Mortal Magic exists BECAUSE of the outer gates. I think it is closely tied to the beginning of human application of magic, but I am not sure whether the door was just left open, or was intentionally opened by one of the attendants of the Outsiders. Once they were part of this universe, but either moved on, or were banished.
Which brings us to the function of the gates, and why they must be patrolled instead of closed. Without the gates, Mortal Magic no longer exists, it functions on the frequency of the Outside via the gates, and because of that, Mortals can directly summon Outsiders. This is also part of the reason that Outsiders can't be harmed with Mortal Magic, or at least, it doesn't do very much damage, as they are of the place where it comes from.
What is Mortal Magic responsible for?Stars and Stones refers to the first humans wielding magic, with only the stars and stones to guide them, and with human belief, imagination, and magic, the Nevernever was born. All gods, major and minor, and all mythological creatures are born of this, and the pantheons will interact with those who wield power. History becomes even more interesting. We have WoJ that the NN extents to everyplace man has been, and the strange geometry of connections between here and there is explained by human perception of connections, not just because places have a similar feel, but because humans created the NN, via a collective belief or at least knowledge in the things that exist there. The fact that sometimes wizards can create "pocket" versions of the NN when they dream leads me to this conclusion.
So if the NN was born of Mortal Magic, and Mortal Magic is subject to the Gates being opened, why are the Gates in the NN?I am not sure, but I think this is a chicken-and-egg question, and that the NN MUST exist to contain the Gates, and as soon as man had access to the magic, the NN existed. With our oldest myths in charge of patrolling it. What better way to use all of the scary boogie-men we conjure up in the dark, than to turn them on those who don't belong here?
Why is the Fairy part of the NN closest?For several reasons, but one of the primary reasons is the interbreeding between humans and Fae, specifically the Sidhe. Interestingly, when Mother Summer takes Harry to the Gates, she mentions how long it would take him to get there himself, so one might speculate that even with the non-Euclidean nature of the NN, Fairy makes up a significant part of it, or at least multiple locations. It does give one pause when considering the asymmetry of the Fairy Courts though.
So what does this imply about all of the NN-derived creatures, and their relationship to Man?They are all DEPENDENT on man's collective imagination/knowledge/belief, which we can see via the Oblivion War. We literally can annihilate denizens of the NN by forgetting about them, cutting them off to Mortal Magic, via the link of knowledge or belief. This is also illustrated by the changing of "masks" mentioned on several occasions in Cold Days, as human imagination, knowledge, and belief shifts, so do the natures of the larger beings in the NN. Odin is our best example, but Harry uses several names for Mother Winter as well.
As a very scary aside, this means that sometime in the future DV, the White Court may sparkle. You've been warned.So who would be exempt from this tether?Well, it seems that there is one group that is exempt from typical magical rules, the Fomor. It has been speculated that they are perhaps related to the "Deep Ones" from the Lovecraftian Cthulhu Mythos, which would suggest they are descendants or attendants of the Outsiders, if we are drawing parallels. This explains why they aren't subject to issues with water grounding magic, they don't necessarily disturb electrical items, or machines in general, and they don't take nearly as much damage from magic, since they are of a power with the Outsiders, making them either resonate on another frequency (and able to shrug off the damage) or because they are born of the same power, and it is like Mab and Ice.
What does this have to do with Harry's status as a "Starborn?"I have very little footing for this, even less that my earlier speculation. I would guess that Harry's ability to throw off the mental whammy, and ability to damage Outsiders will allow him to damage the entire patronage network, including the Fomorians, and every other being affected by the "Possession," Nemesis. And while he was able to hand HWWB4 his ass in a fight, I am not sure that just doesn't send him back to his respawn point, much like a demon would, or if he is off the board entirely. If the latter, this will make Harry hell on wheels dealing with the lower arms of the network. Remember, we have yet to see Harry square off with the Fomor one-on-one, as of the end of Cold Days.
II. The Conflict - Hell's Bells.What will the final conflict be? This is where it gets very thin, as I haven't really considered what the elements and the players of the conflict are. I think the pieces on the board are being moved around to stop an attack from the gates from the other side, at least on Harry's side. So I am thinking the curse "Hell's Bells" implies the final conflict (all hands on deck, if you will), from a Abrahamic perspective. For Odin, I think it is Ragnarok, in fact, most mythology has a final conflict myth of some kind.
We have WoJ that time is running out on the stopwatch, but I think the conflict will be different that what we expect, and the solution will be FAR stranger than we might imagine.
III. The End - Empty Night.Oooo, I love this part. What on earth does "Empty Night" mean?Well, there is a solution to the Outsider Problem, to the Fomor problem, etc. but it involves a sacrifice by mankind. Interestingly, it will be one that most of us won't notice (probably), but it will be a giant sacrifice, none-the-less. The solution is to close the Outer Gates. Period. Ending man's entire history with access to magic, cutting off the Outsiders and their supplicants, and rendering the NN non-existent.
We have EVEN seen this model in operation in the Red Court, with all the old/turned members rotting away, all the newly turned becoming human again, and the older non-turned aging rapidly.
What would a world look like with the NN?Not that different for most people, but for Harry and crew, it would be a decidedly happy ending. Harry no longer has to patrol the island, serve Mab, or kowtow to the White Council. He no longer has to fear the Nickle-heads, the Fomor, Papa Raith, Cowl, etc. As they are most likely gone. He has a human brother, a girlfriend he will age at the same rate with, non-were-wolf friends who he can game with, and can rebuild his business as a regular PI.
Ebeneezer would be gone, though, but his journals would remain. Bob would be gone, but Mouse would be a "regular" dog. Kincaid would be gone, but Ivy would be a normal girl. The Carpenters would have their family back... as many as survive.
It is all-in-all, quite a good ending, but it will be hell getting there, and maybe Harry can take up speculative fiction writing, you know, to make some money on the side.
This is the part where you tell me I am completely insane, and I hope you are right.
