Having played many MMOs, I've seen a lot of endgame activities. I loved some, and hated others. I wanted to share an idea that I think could tie in very well into ESO's lore.

For those of you who have played FFXI, this idea is taken from Dynamis.

Overview

Each province could have a "negative", or parallel dimension, which would serve as a raid. The plane(s) of Oblivion from which Molag Bal launches his assault upon Nirn. These planes of Oblivion would be full negative copies of the capitol city of each province, and would be for level 45+ (assuming a 50 cap). There could also be entrances to three different planes in Cyrodiil, which the factions could fight for control over.

Entry

The raid size limit would be massive. 50+ people should be able to enter the plane, but it would not require that many to successfully complete it. A much smaller raid should be able to get by with farming, but would not be able to take down the boss. There would be a substantial entrance fee... somewhere in the realm of several hundreds of thousands of gold pieces to open the zone to the raid as a whole, not per each player. The entrance fee would be collected beforehand, and an item purchased by a designated person to turn in at a designated date/time. Once the entrance to the plane has been opened, the purchaser could spawn entry tickets to hand out to those who are supposed to go. No worries about freeloaders getting in. The zone would then have a 24 - 48 hour lockout to those who entered and killed any enemy/looted any item.

Battle

In these planes would be the Daedric armies of Molag Bal, which would be densely populated throughout the entire zone, leaving room for resting between battles and for strategic pulling. They would be fights which require a serious effort and coordination. Everyone would have to contribute in order to be successful. These large skirmishes would contain NPC leaders, mini-bosses and eventually a "Mega-boss", which would have to be killed in order to get credit for completing the zone.

The zone would have a timer, which would be extended by defeating the Mini & Mega bosses, and by reaching certain checkpoints throughout the zone. Defeating the Mega Boss would not end the zone, and the raid could continue to farm until the timer ran out. The primary planes would have to be completed by a player before he would be able to enter the ones in Cyrodiil.

Items & Loot

I did mention Daedric Weapons, didn't I?

In these planes of Oblivion, you would be able to get relic/artifact armors, and special currencies and items, which I'll explain after the armor.
The artifact/relic armor would be powerful gear which has the chance to drop from any monster in the plane, but it would have a low drop rate. A fully completed plane would yield anywhere between zero and ~8 pieces of armor. Each piece of gear would be bind-on-pickup, be of a light, medium and heavy nature, and only certain armors would drop from certain planes, requiring a character to run them all in order to get a full set.

The planes would also drop special currencies, weapons and items. These items will be what is required in order to get a Daedric Weapon. However, make no mistake, what I'm proposing will be a monumental task, so that these Daedric Weapons would be kept extremely rare.

The weapons that would drop would be a base shell. A level 1 weapon with no stats at all and absolute junk if you don't intend to go through with upgrading it to it's full potential. This is where the currency and items come in. Each plane would drop it's own form of currency which come in denominations of 1, 10, and 100. This currency is a semi-rare drop and is the fee to upgrade the weapon to it's next stage. The special items that can potentially drop would be the materials.

In order to upgrade your base weapon to it's next stage (there would be multiple stages before completion... 3 or 4 or so), you would need to collect enough of a certain currency and have the special items to turn in to an NPC. This NPC would also be able to take stacks of the 1, 10, and 100 piece currencies and convert them into a higher form. 100 1's into a 100 piece, 10 100 pieces into a 1,000 piece, etc etc.

Here's a layout of how the stage upgrades would look for someone in the Ebonheart Pact:

Quest 1:Required: Stage 1 weapon -- "Special Items" from Planes of Skyrim, Morrowind & Black Marsh -- 4 100 pieces from Skyrim Plane
Yield: Stage 2 weapon

Quest 2:Required: Stage 2 weapon -- X amount of high level crafted weapons of the same type -- 14 100 pieces from the Black Marsh Plane
Yield: Stage 3 weapon

Quest 3:Required: Stage 3 weapon -- "Special Items" from the planes of Cyrodiil -- 61 100 pieces from the Cyrodiil Planes.
Yield: Stage 4 weapon

Quest 4:Required: Stage 4 weapon -- "Special Item" dropped from the Mega Boss of a Cyrodiil Plane -- 1 10,000 piece from the Cyrodiil Planes
Yield: Final Daedric Weapon

As you can see, in order to get the final Daedric Weapon, you would need to put in a massive, MASSIVE commitment of time and money. However, the weapon should be powerful enough as to justify the effort and the item would still remain rare enough so that you don't have every other player in the game running around with one. I'm thinking that it would potentially take a full year's worth of raiding and farming (and huge guild/friend support) before anyone ever got their fully upgraded weapon. As I said, a monumental task so that the Daedric Weapons stay rare.

Can't be restricted to class because every class can wear every armor and wield every weapon. I suppose you could have certain stats that would be more desirable than others (such as bonus to sneak and stamina for the stealthy types) for certain classes, though.

As for the rest of it, I'll have to muse over it a bit more.

yeah the class restriction idea is up in the air right now until we know exactly how the class system will work. Probably should have thought that out a bit better, I'll edit the OP to make it a little more accurate to what we know of the system so far.