Right, but to clarify the earlier two points:
First, being able to play X differently from others doesn't necessarily require a talent system, and therefore the talent system itself would usually be a choice made specifically to not give as many buttons or as much versatility to jobs, replacing potential gameplay with menuplay (usually to avoid 'overloading' the player with simultaneously actionable choices).
For a very, very minor example of this already in game, consider our different rotations on what few jobs have them to any meaningful degree: Optimal Drift Monk, for instance, plays a fair bit differently from Standard Rotation or Double-Solar, and while there is a 'best' choice, it's also the more susceptible to failure (more 'difficult').
Moreover, we easily could have multiple different-feeling macrorotations play out quite competitively over a given fight. Imagine, for instance, if Fists of Earth/Wind/Fire had been made actually decent mechanics (rather than just being one norm and two modest situational buffs outside of unlocking rotational skills) instead of being outright pruned to make room for the likes of Anatman? You could easily have it so that those stances feel quite distinct from each other and that while, yes, there may be one particular intricate rotation between all three stances that'd perform some 0.2% above the next best, you have a decent amount of choice there.
That proposed larger number of in-combat options --taken or left, though the buttons would usually still serve a purpose regardless-- would be essentially a gameplay (choices made in combat) means of differentiation, as compared to a menuplay (choices made outside of combat).
Personally, that's my preference, especially if we ever take a turn towards designing for jobs, rather than designing for role templates+gimmick (which we then call "jobs"). Granted, it can coexist with certain systems of menuplay customization, too.
Second, there's simply a matter of how to source those sub-builds, between either...
1. ...splitting up [some additional] part of the job aesthetic X ways (to follow with the Monk examples... Light Chakra Monk, Dark Chakra Monk, or Hybrid Monk... or, say, Fire/Lotus Monk, Wind/Gale Monk, Earth/Adamantine Monk, etc.), or...
2. ...adding to/atop an aesthetic via something from outside that job (such as per XI's sub-jobs, or by adding to Dragoon the Flowing Strikes trait from Monk [allowing it to more freely combo and to increasingly ramp up in speed], Avatar trait from Reaper [allowing it to go all Ran'jit], Evocation from Black Mage [allowing it to build and spend BotD duration more granularly on Geirskogul], or Blindsider from Ninja [causes Jump instead to just jump up high, instead augmenting your next GCD skill as if used from stealth], etc.
Both work. I just feel that a "All Jobs on One Character" schtick like XIV has definitely lends itself better to the latter. It otherwise comes off as a Chekov's Gun left unfired.