Only issue with it, other than it was already done with Cross-Class, and that's as close as XIV's gameplay will ever get to subjobs, is that if you did give true subjobs, there wouldn't be a lot of skills that really mesh in well with whatever jobs' rotations.

Think about it like this. The subjob system in FFXI only ever lets you have half your level (though nowadays there's Master Levels, but if you notice, at max ML, it lets your subjob be half your iLvl but I digress).

In XIV this would mean doing something like BLM sub RDM. You'd get every skill RDM has up to level 45. This means awesome additions like Fleche, Corps a Corps, and such. But it also means just a bunch of spells that useless to you as a BLM, since they don't feed into your Astral Fire or Ice in anyway, and they have lower potencies as well. If we gain the job traits too, that'd be absurdly unbalanced, as it would make BLM so mobile with the main part of their rotation, they'd ascend into godhood.

Then think of it in reverse. RDM sub BLM. RDM basically gains nothing, since the stronger part of their rotation wouldn't depend on Astral Fire or Umbral Ice. Unlimited MP would be nice, as would Manaward, and the DOT from Thunder III would be nice, but even giving job traits, RDM sub BLM would be a complete weiny compared to BLM sub RDM.

Hell, even when we had just cross class skills, they had to implement balancing so that cross-class'd abilities gave weaker buffs than the main job's (which they already did, they just weren't weaker by a large enough margin). This let jobs like BRD get more ATP and crit rate from their cross class skills than the jobs maining those abilities (DRG/MNK).

So in order to implement subjobs at all, they'd have to vastly rework, likely from the ground up, every single job. Might as well make a new game lmao.