I didn't think the 'Sound' was going to be addressed in 6.0 and in hindsight I don't think it should have been either. It could've been the 'big bad' of the next arc if they'd written Zodiark's demise to slowly unravel the 'laws of nature' whereby we gradually start to see the Final Days begin to creep up around the world with increasing intensity across expansions. One of other complaints about EW is the Final Days, an extinction level event for the Ancients, is arguably no worse than any other calamity for modern mankind.

Additionally, rather than a time loop paradox, the WoL could have started having dreams or Echo flashbacks to the Final Days. I'd argue they're overdue as those less rejoined than the WoL have visions/memories of their past lives. We could've been trained in how to hone our Echo by any one of the unsundered. Elidibus should've procured from the CT to aid us with Zodiark and offer a glimpse into what happened from his POV. We were denied seeing everything from Zodiark's summoning, to Elidibus' mediation, to the sundering.

There's so much more that could have been done. Unfortunately, I think the issue is Yoshi-P was bored of this story (if he ever liked it at all) and wanted to move on to something else. I already had that feeling before the interview where he mentioned needing to be interested in what he was doing. It feels like Ancient and lore fans got the shaft because he just wasn't into it. I also wonder how much of a factor that was in Ishikawa stepping down as MSQ writer. I'd find it difficult to believe EW is what she intended.