I'd add in complex 4 man dungeons with 6~12 bosses, along with a questline revolving around them. The dungeon trash would be difficult too. Call it, "Brutal Roulette" once you've done'em all. It'd reward you with gear 5 ilvls lower than Savage, and if you completed the roulette, it'd reward you 450 <insert current weekly capped tome here> along with 900 of the other two, and 50,000 gil. It'd also reward weapons 5 ilvls lower as well, once a week.

It'd have some difficulty limitations.

One raise allowed per player per boss, and you gain up to 6, one for every boss defeated. Wiping doesn't fail the duty, but your raises are kept separate from dying and returning.

Two hour time limit.

In the 12 boss ones, every boss from 7 and onwards will add 10 minutes to the instance timer.

The boss arenas don't seal. This means you have total freedom inside, for weal or for woe. Though for some bosses, they may gain an insanely powerful buff if taken out of their arena. Also, depending on dungeon design, there may be no way to leave their arena once you are in combat inside.

Titles and a Mount for enough completions of both the dungeons themselves and the roulette.