Given what we have, which could easily have some sort of quirky weird twist that takes some people by surprise ("you thought blue! but I actually wore the shirt to mean lemon daises! *huehuehue*"), I am thinking towards a soul / reaper inspired job.
Thoughts relating to last expansion (ShB) soul magic was a new (sort of) and major magic, the character got a reaper like appearance when it transformed for combat, we've Anima coming (anima relates to souls, by name) and LoooooaaaaDDsss of people dying (or so it looks from the trailer, at least the end of times which reapers are not unknown to be reference to), what appears to be a scythe in the trailer, two reaper t-shirts, while not filth Anima in another FF offspring game referenced her with Fury, technically reapers are spirit guides which gives SE the best way to produce a 'dark / death' related job without it feeling like Necromancer (necromancer being strongly leaned against by the story, so.. while I imagine they 'could' make something up to try and make it fit, it would be an awfully large amount of work when they could just offshoot from the idea slightly). Necromancer a few times has done surprisingly well in polls for being really not a FF thing, and I think something like that would intrigue Yoshida. Some forum users have been requesting a dark theme'd DD job (BLM while has some dark elements doesn't really feel like a 'dark theme'd job', and more so a classical wizard- like Diablo 3 wizard). I feel the reception and creation of Ranjit, for being a minor character, was pretty good (they designed him out of their normal flow, he is inspired design of Golbez and Ravager and has a semi-reaper appearance once transformed). Ranjit also happens to have a sanskrit name which lends a bit of flow to the expansion coming (seeing as we appear to have some areas potentially inspired by some of that). Golbez being this beefy strong boi mage, so having reaper magic (soul / dark) with heavy armor (maiming, at least to our best guess this next job will be in dragoon armor which is the most tanky variant of DD armor). Making a bruiser mage would also allow the job to be a bit more special within the melee DD family, so it's not just another physical job.
Also I feel, at least if it was in the trailer, either as Zenos or the WoL that you have a good opportunity for both visual imagery and FF imagery- either Light (Paladin) vs Dark (Reaper, Zenos) or a bit of both with contrast of evolution with the Paladin going Reaper in the fight (anti-cecil basically, who went from Dark Knight to Paladin).
It's clearly an attempt to line it up a specific way (because a bruiser dark theme'd soul mage sounds pretty awesome), but then my guess is towards a job that is inspired by Golbez, Ranjit, perhaps necromancer (but isn't that job), soul magic, dark theme, perhaps even the moon given our expansion (lunar magic, or we learn something special about the moon), and even might see things like Mystic or Rune Knight concepts. Depending on how far they go on Ranjit (well "if" obviously lol), and if they were inspired by WoW too even, it might start to appear like Death Knight meets Demon Hunter (or maybe like someone who posted the Dante Inferno video game concept).
Or you know it could be a really silly reference to something. . Mascot limited, please look forward to it lol. Hopefully it's not too silly though because I kind of like the game of the t-shirts so it'd be unfortunate if it was announcing something fairly minor.
Also to note if we do the whole 'look inside' concept, one of the shirts has a major album (for that band) called "follow the reaper".