Well it isn't really that they've been keeping tight lipped and more that they changed the class reveal timing from the recent ones. Shadowbringers had the initial class reveal in the first teaser trailer(Gunbreaker, with Thancred showing it, and then they revealed the class in first fan fest) and 2nd in second fanfest. Endwalker was weird because Covid so no fanfest, initial digital fanfest revealed Sage in the trailer(Alphinaud) and then later they revealed Reaper.
But if you look at an even older example, Stormblood, you'll see the same pattern. First fanfest revealed no class, only Yoshi P T-shirt hint(which was a Scarlet Witch T-shirt, which was a hint for Red Mage). The 2nd fanfest they revealed Red Mage, and there was a 2nd T-shirt hint(Spiderman which was referring to the director Sam Raimi > Samurai hint). The final trailer revealed the whole Othard part which had been kept under wraps until then with Shirogane and the main character as a Samurai.
Current T-shirt hint is TMNT which we went over a while ago in this thread. With the theme of the expansion Corsair would be the most likely bet(the TMNT just being turtle, reference to Tortuga). The pattern is interesting because it could also be tied to a larger reveal in terms of the theme of the expansion similar to how Stormblood introduced a whole shift when it revealed Othard. So instead of the whole expansion being just the New World, there could be an additional part tied into it (Meracydia being the usual guess since it's somewhat close and ties into a bunch of the world's lore). No way to know until they reveal more though but the 3rd fanfest might have a lot of stuff, or it could just be more of the same. Delaying the classes does give them something interesting to reveal in the last fanfest if they don't have a secondary location too(first fanfest reveals the expansion location, 2nd the 1st class, 3rd the 2nd class, so it's split somewhat evenly, while if they frontload both the expansion reveal+1st class in first, 3rd kinda has nothing new to reveal).