An Ode to XI?

I couldn't play XI. At all. The only thing I did find interesting about it was sub-jobs. Think it may be possible to bring that back into the "new age" MMO someday?

could be possible, i recall yoshi musing on mmo 'generations' (WOW, guild wars, ff11 being gen 1, 14 and a few others being gen 2, etc) and clamming up on what he thinks a 'generation 3' MMO would look like in some fanfest interviews last year or year before.

Back in ARR and HW, we use to have something like sub-jobs called Cross-Class Skills.
As you leveled a class (not job), some of your skills you gained could be used by other classes/jobs.
Classes could pick skills from any other class, while Jobs could only get skills from 2 specific classes.
For example: PLDs could get skills from MRD and CNJ, WHM could get skills from ACN and THM, etc.
You could only select a certain amount of skills to use though, gaining the option to pick more as you leveled, with 10 being the max at level 50.

They replaced them with the Role actions in 4.0 because it wasn't very newbie friendly and certain skills were mandatory, while others saw little-to-no use.

The thing about FFXI's sub-jobs though was the meta wasn't about wanting to give you access to a second Job's abilities (although that did apply, it was somewhat underwhelming because the sub-job was limited to being no more than half the level of your main, so you could only use that Job's lowest level abilities, there were some exceptions to that, like WAR and NIN being the 'meta' sub-job for tanking for Provoke and Ustusemi respectively), it was about the choice of sub-job boosting your main Job's stats - naturally some choices of sub-job worked better with certain Jobs than others (for instance, mage Jobs as a sub-Job for a melee was a big no-no as it did nothing to enhance melee abilities).

FFXIV already has the stat boost in parties where a Job's main stat is boosted across the board temporarily based on the each Job's role (like, VIT is boosted for all party members if a tank Job is in the party, DEX is boosted for a ranged melee etc) so the FFXI idea is redundant in that regard. And the old cross-class system FFXIV formerly had that allowed the use of one class's abilities with another like FFXI's sub-Jobs do was scrapped also as Mimilu mentioned above, replaced by role-abilities which are automatically learned.

FFXIV has it's own spin of the Job System and so although it will continue to grow and evolve, you won't see FFXI's sub-jobs appearing again here, that's just being realistic.