For me, one of 2 things......

(Unlikely to not exceed 40 hours)
1) Gonna piggyback off of LB, but actual FF traditional limit breaks. I understand XI has 2 hours, but I am referring to a powerful skill gained through mini quest for each job, called Limit Breaks. You can only have one per character (Which may give ppl a reason to lvl alts), 5-ish min recast and is the most powerful DD JA in the game.

Example......

-Ninja may have multiple shadows which fight with them for awhile (They cast, ws, do whatever the NIN does).
-BST may be able to have a chance to catch monsters they or group beat in battle while active, which they can summon later then aka SMN Odin or Alex 2hrs
-Dragoon's Wyvern temporarily ages into a huge Wyvern (Can Surge with it allowing for a Legend of Dragoon look or not.....)
-MNK Super Boost (Temporary Super MNK mode)
-PLD fires a conal light beam from its' shield which melts everything generating massive hate or something.....Just a few examples.

(Likely to not exceed 40 hours)
2) A quick mini scenario were the player goes back in time, visits the Zilart. The reward is a OP AF set for a specific job of your choice which resembles Kam'lanaut battle armor, but inspired by that specific job.