Now I know some of you will immediately scoff and say this will be unbalanced or break the game, but I believe this can be pulled off quite simply and open up all kinds of new possibilities. I ask that you read this with full consideration and open imagination.

The basic idea here is to unlock your lower level (I think up to level 15 would be a good spot) JA/JT's for use across all jobs. The versions of these would be equal to the subjob versions or possibly even less. A menu similar to the BLU spell menu would be used to assign which unlocked JA/JT's you want to use, possibly with point values as well. This keeps someone from having ALL active.

Something obviously this powerful should take some work to unlock. I think it could be accomplished by implementing something along the lines of "Job Mastery." This would entail requirements along the lines of: getting a job to 99, spending a certain amount of merits or filling up a special "Job Mastery" merit skill, and possibly some sort of Limit Break style quest. Completion of this would then unlock your level 1-15 abilities on that current job to be assigned to other jobs. As an incentive and to balance other jobs now having access these abilities they could also award you with some sort of buff on your 1-15 JA/JT when used on their Main Job.

The idea is that you are now a trained Master at using these abilities and that naturally you'd be able to use them in any situation.

Some notable examples on the JA/JT possibilities:

BRD:
Job Abilities:
Access the ability to sing the level 1-15 songs: Like /BRD, you could only have 1 buff active.

BST:
Job Abilities:
*Charm and the lvl 1-15 pet commands: This would probably only amount to being used as a toy or for fun.
Job Traits:
Wide Scan: something handy to have on any job.

BLM:
Job Abilities:
Elemental Seal: Good for any magic user.
Access to lvl 1-15 Black Magic: No big help here but you'd have access to Blaze Spikes, Drain, and Bio.
Job Traits:
Magic Attack Bonus

BLU:
Job Abilities:
Access to lvl 1-15 Blue Magic: Make what value of this you will. A few nice defense buff spells.
Job Traits:
Dependent on spells assigned.

COR:
Job Abilities:
Access to lvl 1-15 rolls: similar to BRD
Job Traits:
Rapid Shot

DNC:
Job Abilities:
Drain Samba
Curing Waltz

DRK:
Job Abilities:
Last Resort: helpful for a lot of DD
LVL 1-15 spell access: Drain and Bio
Job Traits:
Attack Bonus

DRG:!!
Job Abilities:
Jump: A nice extra attack to throw out
*(?)Call Wyvern(?): According to the lore of the game, the wyvern you rescued in the quest is now YOUR pet. Mastery should allow you to call in it whenever you wish. Measures must however be taken so that it doesn't outdo a DRG main's wyvern.
Job Traits:
Attack Bonus

MNK:
Job Abilities:
Boost: Useful for any DD.
Dodge: Helpful to nearly everyone.
Job Traits:
Martial Arts: Speed up H2H a little on any job.
Counter: A lot of use for tanks and DD
Max HP Boost

NIN:
Job Abilities:
Utsusemi: Ichi: A little extra protection for everyone, it's not like they'd have acces to Ni
Job Traits:
Dual Wield: Opens up the first tier so any job can dual wield. Not as exciting now with so many other Dual Wield options but could be nice to try.

PLD:
Job Abilities:
Shield Bash: Give other shield users a little bonus.
Job Traits:
Defense Bonus

PUP:
Job Abilities:
*Activate: Would need to worked out with a similar solution to Call Wyvern

RNG:
Job Abilities:
Sharpshot
Possibly also give access to ranged weaponskills
Job Traits:
Wide Scan
Accuracy Bonus

RDM:
Job Abilities:
Spell access LVL 1-15
Job Traits:
Fast Cast

SAM:
Job Abilities:
Third Eye: Everyone gets a free MISS
Job Traits:
Store TP

SCH:
Job Abilities:
Penury and
Parsimony: Useful for all magic users

SMN:
Job Abilities:
*Access to Summons: would need to be adjusted
Job Traits:
Max MP Boost

THF:
Job Abilities:
Sneak Attack: Useful for all DDs
Job Traits:
Treasure Hunter: Have access to the first tier on all jobs

WAR:
Job Abilities:
Provoke: Would open up tanking possibilities a bit more.
Berserk: Useful for all DDs
Job Traits:
Defense Bonus

WHM:
Job Abilities:
Divine Seal
LVL 1-15 Spell Access
Job Traits:
Magic Defense Bonus


*Alternative Idea For Pet Classes: Instead unlocking the calling abilities, give a JA that calls the pet to perform a single action then disappear.

Alternative Idea to LVL 1-15 unlock: Rather than unlock all of that jobs LVL 1-15 JA/JT's, instead unlock access to a predetermined defining JA/JT for the mastered job to be used on any job. This could be the answer as some LVL 1-15 Job Masteries would produce less-than-stellar results.