First off, there's not really anything I can complain about in Shadowbringers. It's amazing. I enjoy it as much as I enjoy FF9 and there isn't much in life I enjoy as much as FF9 so that's saying a lot.
I have one small complaint with old content; MSQ Roulettes.
No, not the cutscenes. We get really nice XP in exchange for the cutscenes, leave them be. My complaint is that there seems to be quite a bit of difference in XP given between Praetorium and Castrum. Obviously, you don't always win at "roulette" but I think Castrum XP should be brought up to the same amount as Praetorium.
MSQ Roulette is a good opportunity to potentially help new players as well as have a massive incentive via really good XP but I've seen people leaving as soon as they see it's Castrum or going AFK in hopes they will be booted. I don't think Praetorium XP should be lowered.
That out of the way, to Shadowbringer requests:
GNB:
Sonic Break - I love GNB. I'm NOT a great tank as I'm still learning that role I don't play often. One thing I'd like to suggest, partly to hear some career tank's opinions on is making Sonic Break oGCD. It feels odd and a tad clunky having it as a GCD part of your rotation.
Gear:
No. IX/Heirsbane - Given the length of these weapons, would it be difficult or cause problems to give us a replica of these Garlean weapons for GNB? I honestly can't really think of any issue they'd cause being carried on the back.
Crystarium Light Mail - I was hoping there'd be a version of the Crystarium mail like Lyna wears without the pauldrons on the shoulders. Currently, there is not.
Amaurotine Robes - Can we please have wearable replicas of these? I don't mean the Seer's Cowl-style robes, these actual robes.
Lv. 60/70 Primal weapons - This would be a lot like RDM/SAM getting lv. 50 primal weapons.
Minions/Mounts:
Wind-up Lyna
Wind-up Dulia-Chai (many of us love our favorite spoiled mother-figure)
That's really all I've thought of that I want in the near future.