Considering the jobs that are left from the series, i would say that Necro would be a semi-viable option. Lets see some of the jobs that are in FF that havent been touched. Now mind you this isnt a flame or a slam on Valcristiel, its just that he brought up these jobs and they have been brought up before so i'm throwing in my 2 gil about their impact on the game.
Time Mage: Haste, Slow, Stop ect. We already have WHM and RDM for this, no need to make another job for stat effects that are already taken by 2 classes taht are usually required in alot of party setups.
Mystic Knight: (aka from the FFT series "Temple Knight, Holy Knight, Holy Swordsman" and a bunch of different variations) This would really depend on how its carried out. Would they make it like DRK? or PLD? a mix of the 2? If so, how would this be seen in a party? If this was in the game, why choose it over say DRK or PLD?
Berserker: We already have them represented with the WAR ability "Berserk", so theres no need to make this as specific job.
Alchemist: Considering how hard inflation has hit the AH, and considering this job practicly requires to be loaded with items, i dont see anyone choosing this job even if it were availible. What would an Alchemist bring to a party that a WHM can already do for free, or any mage that subs whm for that matter.
Mime: As cool as this job can be, lord knows how far you would probably have to take all the other jobs in order to unlock this. All jobs to 30? Ok cool it wouldnt be too hard to do that. Considering there would be alot of ways to make this job work, its kind of hard to say how it would be implemented in order to make it function in the game. How would they farm if theres no one to copy moves from? Would they have everyones WSs? Would they be able to use all weapons? If they did make this a playable job it would be interesting to see how it would be implemented.
Dancer: From what i remember in FFT, Dancer was the female counterpart of Bard, only their moves were concentrated on the enemy and not the party. Now since Brd songs already have effects against the mobs, why make a whole other class that performs the same role? You might be saying that "Well this means that there is another support job in the game, how can that be a bad thing?" Well it boils down to who is going to chose to play this job. Most likely those who chose to melee as their main wont be taking this past say lvl 15 or so since most dont lvl support jobs anyway. Most melee have their main and then lvl a bunch of subs and if they like one of the subs, they take it further. (now i'm not saying this is always the case but i'm going from personal experience with other people) Ok so this leaves other people who level support jobs. So that means less Bards and WHMs.
Some jobs that I would like to see are Geomancer and, i know it would never be implemented, Chocobo Knight. Yeah i know a mounted knight would sound kinda corney but hey its something this game hasnt seen yet as a specific job. Although there was a way to do it in FFT.