Imagine you plan to go to a cinema to see Spiderman 3, and the guy at the door doesn't let you in because you don't have the ticket stubs proving you have seen Spiderman 1 and 2. Sounds ridiculous? Then why do we accept this sort of thing for our MMORPG end-game? You can't spend your evening in raid dungeon 3 if you haven't been to raid dungeon 1 and 2 first. Either there is some sort of attunement, or you need the loot from the previous raid dungeons to stand a chance in the following ones. To a certain extent the end-game is linear.
A much better system would be non-linear, with no raid dungeon requiring having done another raid dungeon first. You log on in the evening, see who of your friends or guild mates is on, and based on the available numbers, classes, and personal preferences you choose a suitable raid dungeon. There could be dungeons requiring different raid group sizes, and there could be some variation in difficulty. But the difficulty would be in learning the encounters, understanding the raid bosses, and overcoming their challenges, not in accumulating gear, reputation, or attunements. A game based on skill, not on how much time you spent doing prerequisites.
But how could we get to such a system? The easy part is dropping all sorts of attunement, reputation or key requirements. The hard part is coming up with alternative rewards for raiding, which don't come in the form of epic gear which makes your character stronger. The linearity of MC leading to BWL leading to AQ40 and Naxxramas wasn't based on attunements, but on epic gear requirements, with the lower raid dungeons being required to equip you for the higher ones. In fact the whole attunement chain of The Burning Crusade is only there because the epic gear in TBC raid dungeons is less good, and wouldn't be required for going to the next dungeon.
What we would need is all raid dungeons giving the same quality of epic gear, but then give out different alternative rewards that wouldn't make your character stronger, but could serve as status symbol. For example titles like LotRO uses, or armor dyes, or items that add visual special effects to you weapons, or trophies you could display over your mantlepiece in player housing.
With rewards being capped, characters will reach a second power cap shortly after reaching the level cap, the point where they are "full epic", and any further rewards are just cosmetic. With avatars thus being equally strong, there is no more problem organizing raids. And the advantages would also apply to PvP. Any differences between characters would be based on player skill and knowledge, not on the stats of his avatar or the keys and attunements he did. And it would be easy to add new content accessible to everyone, without having to raise the level cap. With added content just adding to your choice, the content would be better utilized, ultimately adding to the longevity of the game. This is something which adding content to the end of a linear end-game just can't provide.