To be fair, they could just put the ROG/NIN tag on Aiming gear and call it a day. NINs already use Aiming right side. Or they could go the ARR route and put STR and DEX on the Striking gear (and Slaying right side accessories) and solve the problem that way. There's honestly zero reason for Scouting gear (or really Maiming gear) to exist. There's the whole "Job identity" thing, but WHM (combination druid and priest), SCH (academic vaguely combined with Green Caller/Summoner-lite and battlefield tactician/military Junior Officer), AST (combination astrologer and, apparently, geomancer), and SGE (combination wizard, scholar, and doctor) all get the same gear. WAR (berserker), PLD (liege knight with a holy theme), DRK (ronin/master-less hedge knight following their own sense of justice with a dab of dark magic/necromancy-lite), and GNB (basically bodyguards/rent-a-cops) also all get the same gear.
The argument for Scouting and arguably Maiming to still exist as distinct gearsets is just silly at this point, especially with "...of Slaying" already existing and "...of Aiming" already used by NIN on one side.
And before someone says "NIN's don't use ranged weapons/aim at things!", DANCERS don't really, either. Yet here we are.
That said, I suspect you're right. The most likely next Jobs I'd expect would be something that uses Scouting, something that uses Aiming, and something that uses Casting.
Of course, they could just shift NIN into being a "Ranged" Job (a bit counter intuitive, but RDM is a Caster that uses some Melee, so...) and then introduce a 4th Caster. But they may be going for 6 Melee, 3 Ranged, 3 Casters to reflect the 2 Melee, 1 Ranged, 1 Caster paradigm they want for the standard group. Of course, at that point, we'd need 6 Tanks and 6 Healers, and with the mess current healing is, I can't imagine they want to add any MORE (though they could give us Diurnal/Nocturnal again and call that one "new" Job in that sense...)
EDIT:
I'm not quite sure how this would work. The Devs are pretty good about not removing content from the game. Not ALWAYS, but ALMOST always. And a lot of people like BLU as it is today. The only way this would work is if they allowed BLU to level to the level cap of other Jobs, and you had two spellsets you could swap between a set for "normal Job" play, where it would just be something akin to a BLM or RDM with reskinned spells, and then a second set for the Limited Job version, where it's effectively treated in the game code as a different Job branching from the same base (think how SMN and SCH come from Arcanist). When you que for content right now (say as a DPS with a long que), you can swap to other Jobs or even DoH/DoL, but when the que pops, it says the Job you qued as and you have to swap to it to accept the que.
Basically, they'd have to leave in current BLU and just let you shift to a "standardized BLU" to do actual content, and "True BLU" wouldn't be able to que for content above "last expansion's" level.
...I have no idea how they'd program that, but just removing existing BLU is probably not going to work. When they do that with far more standardized Jobs (MCH rework, MNK rework, majorly with SMN rework), every time they do it, it infuriates tons of people to the point they'd be better off just adding a new Job. Like new SMN a ton of people love, but a ton of people hate because of what was taken from them. If new SMN had been added as a new Job with old SMN being a separate Job, even with a new name (e.g. Green Mage), people would have been far less angry about the change.
I can't see them dumping BLU, especially after spending all this effort with new spells, level cap raises, and entire content made just for it/Limited Jobs. They may never add ANOTHER, but...