Preamble/Overview
I would truly love to see the Western Skyrim zones released as a single large pvp zone, where players zone in and choose to either play on the side of Reachmen or the side of the Nords (irrrespective of their chosen alliance). I don't know how legitimate, lore-wise, it would be to have DLC zones where "we" slowly take back the Reach, given how in this part of the timeline it is ruled by the Despot of Markarth, but I would dearly love to be able to run around some of my favourite areas from Skyrim, and I think it has the potential to be an interesting mechanic. It would also be an ideal way to encourage small-group play that is so discouraged in Cyrodiil by the prevalence of zergs.

Essentially, I am thinking along the lines of the monster PVP that existed in LotRO. Players can port into the map at any time, and either choose to be Reach, Nord, or Don't Care. The Don't Cares can be dynamically assigned to equalise numbers if required, but Nord by default. There could be achievements, titles and dyes, etc, associated with both sides and players could choose freely to play on one side or the other, with a 1hr timer for switching. Numbers should be carefully managed so that Reachmen players do not outnumber Nords (since these will just be regular players).

The Reachmen
Reachmen are, essentially, monsters, with unique abilities. They should be, in my view, a bit overpowered to account for the fact that no one likes Reachmen because they stink of troll fat and live in the mud they will probably have smaller numbers (or should have). Increased rank in Reachmen skill line allows one to unlock different kinds of Reach monsters that they can choose to play as. For example:

1. regular Reachmen/Forsworn (who can form regular groups of no more than 12 players and will comprise the bulk of Reach players). These are the first unlocked unit. Groups led by a Briarheart get offensive bonuses against enemy camps.
2. once certain conditions are met, players can play as undead Briarheart soldiers.
3. eventually, with high enough reputation, players can choose to play as Hagravens. Although squishy, they can form groups with up to three Briarhearts, who gain combat bonuses from being in proximity to the Hagraven, and take a portion of the damage from the Hagraven shared amongst themselves (so the Hagraven is basically a buffer and debuffs the enemies, the Briarhearts protect the Hagraven and deal damage). Hagravens can't group with regular Reachmen, but regular Reachmen can benefit from the debuffs applied to enemies. Hagravens can help defend Objectives by summoning giant plant tentacles, but can't group or otherwise join combat if they do this.
4. players could also, by obtaining certain achievements, unlock the ability to play as actual monsters such as frost trolls, minotaurs, giants and mammoths that can roam in certain regions where they can basically act like wandering raid bosses. These could be time-limited play to, say, 2-hour slots, and number limited on the map so that we don't have a map full of mammoths. Players who qualify to play as one of these monsters could be randomly selected according to whoever is in the map at the time, and offered the option to switch from their current character to a monster. If they choose not to, the game selects another qualifying player at random.

In the same way that Hagravens are sort of leaders for the Reach, players on the Nord side would also be able to unlock skills that provide equivalent ranks, from footsoldier, to Housecarl, to Thane, according to their glorious service. Bonuses to leading groups as one of the higher ranks would apply in the same way as for the Reach. A special class of "scout" could exist on both sides. They would have bonuses to stealth and movement speed and have the ability to spot traps. They could receive quests to submit reports that would uncover the fog of war on locations they have scouted for all members of their faction.

PVP currency
Reputation is the currency, and this can be gained for each side by capturing, building and defending Objectives, leading raids against enemy camps, killing enemies etc. Reputation can be used to purchase skills and abilities (for the Nords), and monster classes (for the Reach). Reputation can also be used to purchase small houses (such as Skyrim-style crumbling forts, a tower, a dwemer ruin, a nord tomb) provided certain other achievements are obtained, unique furnishings from each side, costumes or style pages etc. Basically in the same way as AP can be used currently.

Objectives
Objectives should be smaller than the keeps of Cyrodiil, and given the terrain, siege weaponry is not feasible (except arguably hot oil). Instead, Objectives should be defensible positions (e.g., clearings - on top of small crags, inside gullies, small ruined forts and towers) that are marked with glowing markers to indicate they can be captured. They can be captured by building up defenses around the position, such as log spikes, bone chimes and pit traps. Pit traps can be used to "capture" Nords. If a Nord is captured in one of these traps, if there is a Hagraven nearby they can turn them into a Briarheart to force them to temporarily fight on the side of the Reach (10 minutes or until death?). If they die, they respawn back on their own side.

Fortifications
The longer an Objective is held by one side or the other, the larger it has the potential to become, with tents, campfires, etc and NPCs gradually increasing in number, depending on how many resources are put into it by players (stone, wood, leather etc - since leather is so readily available and basically worthless, this would give it some value). For example:

1. As soon as captured, a firepit appears in the centre. This is the interactable that lets players build up the fortifications.
2. For the first 10 minutes after capture, players can build log spikes (to circle the camp, let's say this takes 20 logs) by adding logs to the firepit. This can be done from the quickslot bar, in the same way as repairing a keep door. Each stack of 5 logs makes one "fence" of fire-blackened sharpened fence poles.
3. In the first 30 minutes after capture, players can add food to the fire (let's say fish, since they are also worthless). Each stack of 10 fish generates one NPC to patrol inside the radius of the camp. Players can also craft traps when inside the camp that can be placed within a certain radius outside of the campfire. (tripwires, bone chimes etc). The number of traps that can be placed depends on the size of the camp.
4. After the first 30 minutes, every hour that the camp is held, increases the potential size. At camp size 2 (after the first 30 minutes) players can double the size of the camp by adding double the number of logs (let's say 40). At this point, leather can also be added to create tents. Tents can house up to 5 NPCs, so at camp size 2, 1 tent can be placed (requiring 10 leather), and more NPCs can be generated up to that maximum by adding more fish, as before. And so on. NPCs will defend the camp and give defensive bonuses if players fight with them.

If your enemies capture your camp, they can choose to either raze it to the ground (preventing it from being recaptured by you for some length of time), or leave the fortifications in place and add their own on top.

Y tho?
What is the purpose of Objectives, I hear you ask? Well, recall from Skyrim that the map also contains numerous ruins, mines and logging camps. Each of these could have a certain number of potential encampments in a certain radius of it. If a faction "owns" a threshold number of these camps, they have enough numbers in the area to hold it, and can access the ruin, or slowly gain materials from the mines or logging camps to build their fortifications in that area. Camps can only automatically benefit from these resources if they are adjacent to them, or have an uninterrupted link to a neighbouring camp. Therefore, players capturing objectives held by the enemy need to make careful strategic choices about whether it' more important to hamstring the enemy or boost themselves instead.

The map
While the Cyrodiil map has up-to-the-second news about where all the fights are happening, this is not terribly realistic and leads to zerging. I therefore propose that the map of Western Skyrim is covered by fog-of-war. You can discover Objective locations to unlock them on the map, but will only know who holds them if you actually go there and scout them. Given that the map is large, and the Objectives themselves relatively small, there should be a much larger number of them than seen in Cyrodiil (> 30). Capturing them should be a simple matter of being present for a few minutes, like capturing the flag mechanic in Cyrodiil. There is also potential to unlock the city of Markarth, either as a raid or as an IC-style sub-map, in the same way as was done with that desert raid in vanilla wow (I forget the name). Perhaps, whichever faction holds the gates of Markarth gets to assault the city, whilst the other side defends it.

Anyway, just a few ideas - let me know what you think, what you like, what you don't like, etc.

EDIT: For clarity, I just want to mention that when I say "Western Skyrim", it is not my place to decide what qualifies as Western Skyrim, but I would imagine that a map like this would not cover all of the remaining Skyrim regions. Since the Despot is based in Markarth, that is where I would imagine the Reachmen would be based, and so I would like a map that covers the Reach, perhaps the north/west part of Falkreath hold (boundaried by the Lake, perhaps), perhaps as far to the east as Rorikstead in Whiterun hold, and a bit of Southern Haafingar and western Hjaalmarch.