The one thing I think is missing, and probably needs to be added in some way is content that is middle of the road content, not end-game, but is challenging (*). Content that does not rely on scripted instant death or enrage for difficulty. Content that is forgiving enough that skilled players (skilled != elite) can recover if they do the right things. Content that is variable enough that there is no single way to defeat it - I get so tired of robotic demands to watch a video before playing. Above all content that is fun to play, and remains fun as players increase in power.
( * - Note: Challenging does not mean hard enough that only a small minority of players can beat it, it means that most groups can beat it, but it's not a cake walk for any but the elite players.)
If Boss battles and other content was not exactly the same every time with the same moves, phases and pattern; I think players would find it more challenging. Not only that, but they would not burn out so fast since it would be impossible to blindly speed run the content in exactly the same way each time.
I also think that in addition to the level sync that is applied to some dungeons, all dungeons past 49 should include a small amount of damage scaling and/or Monster level increase (in addition to basic level/gear sync) to compensate for the sometimes ridiculous overgearing in content. Doing that should also lengthen the life of content because content that can't be steamrolled the same way every time, becoming more difficult in line with the power creep of the player, cannot simply be rushed and farmed into the ground.
By all means have end-game content, by all means have farmable content, but let's have more middle of the road stuff that is fun to do and which you don't burn out on.
Procedurally generated maps (or portions of maps), some level of random variation in mob/trash/boss selection and placement, changing mechanics and so one, along with slightly better AI in the Bosses and trash would go a long way to increasing the longevity of all content.
That kind of content is not easy to make, and requires a lot of testing, so it may be a pipe dream, but I think it would help the game immensely. I also think that occasionally adding in a new dungeon (using the ideas outlined above) below lvl 50 would be very helpful, especially if it were properly level sync'd to maintain it's difficulty level.