An idea (almost) straight from Ragnarok Online; allow us to "transcend" jobs. By this, I mean you level to 99, beat a solo bcnm of some sort like the old Maat fight, then you are level 1 again on that job. EXP requirements double or triple to level, and the final outcome is slightly better combat/magic skill and slightly higher base stats. To make it slightly more appealing, add a special job trait/ability/spell the second time you reach 99.
Transcending can only happen once per job (i.e. you cannot level to 99, transcend, level to 99, transcend, etc), maybe capping the amount of jobs you can transcend at 2-3.
On the same note, perhaps nerf Abyssea exp for transcended characters until ~90 so it forces people to repopulate a lot of the older zones to do fields/grounds of valor.