Well here is the thing dailies and quests are one type of content that we can get involved in another type could be a randomly generated dungeon.
*No this is not what we have now.*
The one basic dungeon WOW is missing is a dungeon like those in older games where the content is derived randomly.
1. Take a basic cave with lets say 4-5 rooms in a maze pattern.
(You generate this on the fly every time.)
2. Have randomly placed bosses from all the dungeons that exist in the game, this way no new bosses are needed to be designed.
Randomly generate mobs from 3 to 6 in a group.
3. Dungeon is only available at level cap, no experience/rep is given in these dungeons.
4. Rather than giving "gear" on being defeated these bosses will give gems, gold, crafting mats, pets, mounts, and unique quests that are level appropriate.
5. Quests given by the bosses lead to items that can be cosmetic or even boa in nature.
Again the "Infinite" and "newness" factors of such a dungeon would never go out of style, pets and mounts, and unique items can be added every expansion without detriment.
Where would you locate these instances?
You could put entrances outside of all the major cities, or places like the faire.
This will keep the old content alive.
The dungeons would have a theme similar to their location, for example make such a random dungeon in Darnassus,
it could have a cave structure which looks grown or covered by roots, in a hollow of a tree.
A dungeon below Dalaran might be made of ice and have titan structures.
In Shat you could put a dungeon that looks like a sewer with the strange Draeni designs.
Again the design of the dungeon would be generic enough that it would just be a quick maze with a couple of rooms.
The art could be recycled from other dungeons, wall patterns, stairwells, whatever you can put in that fits with the city that its near.
However it needs to be generated randomly so that we get that "oh crap" what will happen now feeling, a simple open structure with no doors or anything just a place to explore and fight mobs in.
I atually think thats a pretty good idea aswell. However it would take a bit of man power away from other aspects of the game to revamp, and rescale bosses so that they are the appropriate difficulty. Blizzard doesnt usually try to implement half assed ideas (note i said usually, but i can think of a few things that were implemented that they probably should have thought about a bit more) It would also have to be updated by new major patch or expansion, which would also take a lot out of Blizzard's resources