The subjob system from FFXI was very very good and 100% should be used in some form in future mmos going forward. And for those saying everyone played the same subs. Yes they did in leveling/merit parties. But in endgame subjob was completely situational and all top tier guilds would expect members and new recruits to have multiple subjobs leveled before getting invited. What people went changed depending on the world boss, dungeon, instance and many times what your role was in that specific situation (and I do not mean role as in just dps, healer, tank).

With that said it would not work in FFXIV. It worked in FFXI and it works in games like Archeage. Because those game make a boss and say go at it, figure it out yourself. There is no focus on "Balancing it", no scripted fights, no specific requirement to enter an instance, no specific time limit to kill a boss (Some soft enrages but like 1-2hrs). FFXIV is all about precision mechanics, perfect balancing, extreme scripted fights, and hard DPS checks. FFXI was focused on depth and complexity. FFXIV is focused on extreme ease of use and simplicity. FFXI was easily on the "Hard Core" mmo side of the spectrum. FFXIV is probably the most casual AAA mmo on the market bar none. The games design philosophies are just polar opposites of the spectrum.

Most things in FFXI will not work in FFXIV because of the above. The game designs are counter to each other. The only thing the 2 game share in common are character models and the name Final Fantasy. So while I 100% loved the subjob system no it should not be in ffxiv because it does not fit.