Anything that helps DRK to get his gameplay identity back is good to me, now to the questions well:

1º My issue with 5.x DRK is that is no longer DRK, it's just a simplified WAR using DRK animations and a few extra oGCD every min, no complexity, no job mechanics cohesion since everything is basically independent keep on recast and useless mechanics/gauges on screen aka the whole Darkside stuff, DRK deserve have a gameplay that don't remember at all any job already present in the game like it did on the past not what we have now.

2º Of couse there is, rework the whole blackblood system that is basically WAR's Rage system copy, rework Delirium and repurpose Bloodspiller, Quietus and Living shadow (this one to make Darkside gauge representation mean something at all) and create actuall Darkside management by ajusting Edge/Flood with other skills so the whole mechanic doesn't run on auto-mode, make DRK mechanics interact with each other, make them feel they have a purpose and all of them build somethign together like every other job in the game.

3º Everything, his combos, his MP management, his APM and speed, his mechanics interactivity betwen resources, GCDS and oGCD and his animations, yeah DRK in 3.x and 4.x have his own issues but the job was engaging, was unique and outside of the numbers and passive buffs balance the job was fine and fun.

4º Well i never liked light themed jobs, my love was always the darkness and DRK was going to be my main the moment they announce it, the job mechanics and the whole feel the gameplay experience it offer back in HW click on me so hard that DRK make my FFXIV experience one of the best thing i ever had, no other job has click so deep like DRK did even today, SB preserve in some way that feeling so thats why i keep main it despite the bad place the job was in terms of performance compared to the other tanks but SHB just totally killed that click i haved with the job and FFXIV has been dying really fast for me making the whole expansiona pain to endure in order to keep suporting the game as i offer feedback hoping to be listen so i hope my boy come back on 6.0.

I just hope all the threads abd feedback mean something at the end.