I think if they're going to add new jobs, they'll probably be more on the tank side if anything. The way I see it this game has enough DPS right now. Samurai and even Dark Knight I can easily see as potential tanks.

-Dark Knight would have absorption and threat abilities with some enfeebles tossed in to help keep HP up and attacks weakened.
-Samurai could have a unique counter attack system with enhanced parry and evasion.
If they want to add yet another healer, can't go wrong with Red Mage, make it strictly healer/support and not dps/hybrid otherwise you'll have too many people play the job for dps/support only.

I'd like to see Rune Fencer introduced, but I'm not sure how they would go about that.