Well, I wouldn't mind some system to help practice specific mechanics. E2S for instance.. All the mechanics are spread through the fight and don't repeat much, so my skill level drops as we get further in. I've only done Quietus a handful of times and can barely keep my DPS up through it. I don't really see how that's going to change much since I have to go through a 10 minute fight to even see it.
This actually makes me appreciate E4S more, since all the hardest mechanics are front loaded. On the other hand, I've never even seen P2 of the fight..