I'm not sure what they can do to make 6.4+ anything interesting, they have no Eureka/Bozja like content in the development, unless they do a big change to how variant/criterion functions, to give it the deep dungeon-esque, "rogue-like, variable and repeatable" gameplay content/rewards, which this expansion desperately needs more of.

I'm looking forward to how maybe the raids might be, and I wonder how they'll even bother to make the Extreme worthwhile, with relic weapons around.

I'd like to be pleasantly surprised by this live letter but honestly I'm expecting little to nothing, since its too obvious by now that they are focusing all their development resources on ffxvi or maybe counting money from their moggy station items.
If they address some issues or reflection regards of this expansion, so it at least feels like they are listening to concerns, it might be easier to look more hopeful for the future patches. But at the moment it very much looks like there won't be anything interesting/spectacular until everything's done with their other ff project, which is kinda sad. :/