"How to mess up a Job reveal?"

Don't tell people what Role a Job is until super late, even knowing that lots of people think it will be a different Role. As a Healer main, I'm still salty about how they knew people thought DNC would be a new Healer Job, and instead of early on saying "It's going to be a DPS Job", they played it out, kind of stringing people along, until the reveal. DNC had the single worst Job reveal reaction in all of FFXIV's history, and I suspect that was the reason for it.

Be clear up front it's a new Tank/Healer/DPS and if DPS, what subrole.

All things considered, it will PROBABLY be a new Caster, since that makes the most sense. Over the years, it's been like this:

1.0: Game start: 2 Tanks (PLD/WAR - once Jobs patched in), 1 Healer (WHM), 2 Melee (DRK, MNK), 1 Ranged (BRD), 1 Caster (BLM)
2.0: Game restart - that said, we got a new Healer (SCH), Caster (SMN), and Melee (Rogue): 2T/2H/3M/1R/2C
[Note NIN was not added until 2.4, but by the end of ARR, this was the count, not counting Cross-Class non-Job builds.]
[Note here that Roles were considered only Tank, Healer, and DPS at this time, though you needed one of each DPS type for the +5% party damage bonus.]
3.0: New Tank (DRK), Healer (AST), Ranged (MCH): 3T/3H/3M/2R/2C
4.0: New Melee (SAM), Caster (RDM): 3T/3H/4M/2R/3C
[Note that BLU was added in 4.5 as a Caster, but has been a Limited Job its entire run, and so doesn't count in a more general sense - and can do all roles anyway.]
5.0: New Tank (GNB), Ranged (DNC): 4T/3H/4M/3R/3C
[Note here, BEFORE this, Melee, Ranged, and Caster were not considered distinct; they were explicitly made distinct, even in your Jobs page of your Character sheet, in 5.0.]
6.0: New Healer (SGE), Melee (RPR): 4T/4H/5M/3R/3C

The most in need of an addition are Ranged and Caster, and of those, Caster has gone the longest without an addition.

The most over-represented is Melee, meaning no new Melee need to be added. Also historically (not that that's ALWAYS a good guide), Melee have been added in every even numbered expansion; 2.4, 4.0, 6.0.

Given all this, the most logical additions would be Caster, then Ranged, then Tank, then Healer, then Caster, then Ranged, and at that point all would be even with 5 representatives.

.

Of Caster, Green Mage (status afflictions; DoTs in FFXIV terms), Geomancer (as a Caster using nature elemental magics rather than a Healer; so if CNJ was a DPS Job, basically), or Time Mage (Green Mage, just with a bit more support and some Comet/Gravity magic; it and AST step a little too close to one another, though, but I guess GEO would with WHM or more CNJ/leveling WHMs...) are the most likely options.

Then again, they do curveballs from time to time. For all we know, they could add Runic Knight as a Tank or something. Just depends on where they want to go with the expansion and what story they want to tell.