I wouldn't get too excited about it ahead of time. Nobody's said that it's going to be free and for everyone.
This feature (increased number of glamour dresser slots) has only been mentioned in an article (with reference to talk to Yoshi-P), but is not listed on the official expansion site.
Do you remember what happened in Heavensward in a similar situation? Then players have been made to believe that they would get access to more retainers with the release of the new expansion. In that case it has also been mentioned only in an article (with reference to talk to Yoshi-P). They indeed added two more retainers, but only paid ones (available only for additional real money monthly fee to those who want it).
In Stormblood it was different. Expanded item inventory was advertised right on the official expansion site front page. And this feature has been made available to everybody and for free.
The reason why I'm so skeptical about it is not only the mentioned case with Heavensward, but also the fact that expanded glamour dresser space is a major feature that asks to be advertised on the official game expansion site. It would build up a lot of hype right before the expansion release. Why is it not there? I believe that there must be a reason, and it could be that this has been made to avoid accusations of false advertising and possible legal consequences (refunds because not all promised features are delivered). And mentioning in press, on the one hand, still allows to build up hype (although not so efficiently as mentioning on the official expansion site), and on the other hand does not entail legal and such big reputational consequences.
Here the situation more resembles the case with Heavensward, which makes it wary.
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