Incentive to tank 50/60? Hmm, helping FC mates out seems to work a fair bit as motivation according to my chatbox...
Outside that? Dunno, anything you give as an incentive reward will eventually become useless for motivation. Perhaps the most long-term useful things though would be either very nice mound of Gil, or instead of XP, an XP token redeemable for XP on any unlocked class the character has.