The current class to job system is pretty nice. I could see having a DPS based Marauder -> Dark Knight (DK) work if you transformed enmity into another stat.
For example, give the DK a pool called hatred which is generated in the same ways that enmity is generated, so the job's pools become Heath, Hatred, and TP.
The new resource could provide for some interesting ability reuse, ex. abilities that traditionally used MP (flash, cure, blizzard ....), could use hatred (or a combination of Hatred and TP) instead, with hatred transforming the ability, ex. cure becomes absorb health, perhaps flash becomes fear, maybe blizzard becomes paralyze ...
As a result of the hatred pool, in addition to the normal priority based gameplay, the player could elect between doing weapon based damage and building hatred to perform hatred dependent abilities.
Then depending on the form the new hatred based abilities were given, and whether or not you enmity generating abilities generate hatred or hatred + enmity, you could turn the Dark Knight into a DPS/something (debuffer, controller ...) hybrid or a tank.
Anyways, I'm hoping and expecting to see a lot of prior FF classes/jobs make their way into XIV and it will be interesting to see Yoshi and his crews takes on how to bring them into XIV.
