Something I would like to see is instead of mechanic on top of mechanic on top of mechanic all happening with an overlap effect where you are sometimes just running from Point A to Point B to Point C doing mechanics is this:
1) Simplify the number of mechanics to an easily memorable amount (players should not have to study like its a 100 question test) I would say no more than 4-5 mechanics
2) Slight increase in time between mechanic executions, as a pay off to keep the challenge alive, make the mechanics even less forgivable (as in you mess up, you die)
I would say that when designing content they should take into consideration that roughly 40%-50% of their players should be able to clear it while it is the newest content. 50%-70% cleared roughly three quarters before the next new content, and finally 70%-85% cleared by the time the next wave of endgame content arrives. I understand the Echo is what helps them achieve this in terms of get that clear percentage but based on what is seen on the game and on the forums they are no where near the percentages I've listed.
A good example of content they did that was good was when Titan EX first came out, it was challenging but fun, when you cleared it you had a feeling of accomplishment but it wasn't impossible. Now a year later and Titan EX is pretty easily completed (which is how it should be as the content is older).
An alternative is this: If they want to have fights with 5-15 mechanics all piled in they need to have check points. And since most of the 'phases' change dependent on the remaining HP of the monster this is entirely possible. HOWEVER, I feel the checkpoint would need to be executed only when the party has shown they can achieve reaching that phase consistently. Basically in application it would be something like:
(Using T9 as an example since its the popular one)
If the party can reach the Heavensfall phase of the fight 3 times consecutively they will be granted a checkpoint wherein they will start the fight with Nael at the max HP for that phase (60% or so iirc) and they will continue to start there instead of from the beginning. This will do two things:
1) It will make Rage Quitting less frequent.
2) It will prevent "battle fatigue" from happening as players won't be stuck in the same cycle over and over and can focus on learning the new mechanics.