As I was reading the reply a few things hit me that might help with the need to redo content over and over.

* Add Weapons via Achievements.

After a certain set number of runs against any one Primal you'll earn an achievement that gives you a pick of any single weapon of your choosing. Example:

Defeat Ifrit 50 times. - Reward: 5 Inferno Totems
Defeat Ifrit 200 times. - Reward: 1 Inferno Blessing (Allows you to choose 1 Ifrit's weapon of your choice.)

Defeat King Moggle Mog 80 times. - Reward 5 Kupo Nut Charms
Defeat King Moggle Mog 300 times. - Reward: Moogle's Blessing (Allows you to choose 1 Moogle weapon of your choice.)

The number of times are just an example, but if there was a guarantee to get at least one weapon by the time you reach a certain kill count then there's something to look forward to.

* Special Items via Primal Achievements

Defeating the Primals any number of times could yield special items. Maybe a piece of headgear modeled after the primal. A helmet with a set of horns just like Ifrit's. A cap with Moogle ears and the ball. An actual feathered headdress resembling Garuda's plumage.

* Increased Drop Rates for Primals

Killing a Primal for a certain number of times will unlock a special drop rate modifier via the achievements which will increase the chances of obtaining something other than dark matter from a chest. It works only for that primal, and the more times you defeat it the better chances of a drop.

Alternatively there could be a special modifier added instead which slowly increases based on how many times you successfully defeat the primal. Any battle against that same primal resulting in your party's defeat will reset the drop rate to it's default value.

* Conditional Achievements

For those who consider themselves hardcore special conditional achievements could be added to existing Primal battles or dungeons. They exist for some areas already. For example...

- "Old School" - Defeat Ifrit Using Only Classes.
- "Pipin' Hot" - Defeat Ifrit In 15 Minutes.
- "Surviving the Lord of Inferno" - Defeat Ifrit Without Anyone Dying.
- "Cold Day in the Seven Hells" - Defeat Ifrit Wearing Only a Reindeer Suit.

Maybe special titles could be unlocked, or even items. The above is only an example.

* Double Your Odds

This idea I kind of like... Upon the defeat of a Primal you're given the option to accept the current chest rewards, or refuse. If you refuse it gives a bonus chance that the chest will have better drops, or odds of an additional drop upon opening the next time you win. This effect can be stacked up to 3 times before the chest is automatically opened and rewards are obtained.

The same could be done for dungeons, pass on the current rewarded loot from the chest or leave it and allow it to increase next run.