The 4th Tank.

[Giantbane]

easily implemented. Bluemage learns his "enemy skills" actions from the job quests at each level interval. It'd make an awesome tank. Aggro Combo could be Goblin Punch>Thrust Kick>Asuran Claws, OT/Debuff combo could be a ranged, mp based like Magic Hammer(MagicdefDown)>Acid Splash(PhydefDown)>Bomb Toss, then an aoe aggro combo Aquabreath>Electrocute>1000 Needles. Mighty Guard, White Wind, Vanish, all cool CoolDown potentials. Vampire Kiss could be a cool dot/Bloodbath hybrid, the possibilities for Bluemage are endless really.

It can really go much further than that. Give them an ability called "Learn" that will reduce the damage of the next ability he's hit with by 30% and give him 1 charge of that ability. When an ability is learned, the "Learn" icon switches to "Cast" (or something like that), much like the AST's Draw ability changes to using the card. You get 30s or so to send a scaled down version of that ability back.

That's in addition to permanent skills like Mighty Guard.

[TouchandFeel]

I want to see Mystic Knight.
Tank class wielding a double-bladed staff that utilizes elemental stances/alignments (Earth/Air/Fire) and swaps between them to enable different abilities and swap out the additional effects of abilities like the finisher in their melee combos.
Their animations should be somewhat acrobatic sort of like MNK and their job specific outfits should have an exotic look to it that is a mix of far and middle Eastern, with the helmet being an armored turban with a veil/mouth-cover as a throwback to their classic look.

[Duelle]

As much as I love BLU, it probably won't make an appearance in FFXIV no thanks to how it is setup to have a set number of abilities and traits on all classes.

Still needs a lot of cleanup, but I just want to prove BLU can be in this game.

I wouldn't mind the 4th tank being based on a similar model to WoW's Brewmaster Monk or Guardian Druid. Evasion based using either a huge health pool to mitigate bad streaks, with self-heals to supplement healers. Or a shunted damage pool that absorbs a percentage of damage taken, which then slowly bleeds the excess damage back to the player. Dancer could work as the latter, Beastmaster as the former.

Personally, I figured SAM would get something akin to the Brewmaster's Stagger mechanic. It just seems like a natural fit since samurai armor (traditionally lamellar stuff) isn't as sturdy as plate and if we were to go by real life analogs, SAM may end up with lower native defense compared to PLD, WAR and DRK. Some adaptation of Shuffle would work for Dancer if you want to make it a tank, though you'd need additional mechanics to make it work.

[Chana]

RUNE FENCER !
Weapon : 2 handed shielded sword (have a blocking rate like shield)
Sub : DRK / Warrior

Combo DPS with an 10% bonus to damages for 30sec -> 2 stacks of elemental buff
Combo enmity like the others + restore small MP-> 2 stacks of elemental buff
Combo Reducing evasion of monsters and mind+ Restore small MP -> 2 stacks of elemental buff
Buff for magic reduction 30%
Buff for physcal reduction 30%
Buff augments evasion 50%
Stance DPS : Add accuracy and 2% damage per stacks of elemental buff
Stance Tank : Add enmity bonus, each stacks give 4% damage reduction, reduce damages by 10%
Buff : convert next magic attack to HP
OGCD : Give 5 stacks
OGCD : Convert stacks into HP -> 250 potency per stacks
OGCD : Convert stacks to damage 100 potency per stacks
Buff : Transfert stacks to party with same bonus as the actual stance
OGCD : Lose stacks, exchange your ennemty on target with the highest member of the party.
Buff: enhance blocking rate, cannot parry
OGCD : silence
OGCD : Convert MP to HP
Spell : Stoneskin
Spell : Regen II (half mp)
Spell : Magic reduction to party 20%
OGCD : Convert stacks to MP and TP (10% perstacks)
OGCD : Stun + magic DoT

[Ipkonfig]

I'd agree with Rune Fencer, give a system similar to Mudra so you can weave runes. If you start at 30 you could make it like FFXI where you can only use 1 rune at 30, then 2 runes at 50, finally 3 runes at 60. I don't think it could be as intricate as it was in FFXI, but just make 6 total runes and different combinations make abilities do different things.

[Chana]

also can do completly different tank with this :
Can stack 5 runes, runes are out of GCD, no recast. Oldest rune wears of when 6th one used.
30sec per runes.

Ignis : Adds 5% damage
Gelus : Adds 5% magic reduction
Flabra : Adds 5% skillspeed
Tellus : Adds 5% physic reduction
Sulpor : Adds 5% critics
Unda : Adds 5% regen
Lux : Adds 5% accuracy
Umbra : Adds 5% enmity

Regulus : Exchange runes for MP 10% per runes
Revilis : Exchange runes for TP 10% per runes
Ennarva : Exchange runes for 30% damage reduction for 30sec for party (physic and magic)

That would make a totally new type of gameplay!

Umbra + Lux : attacks are conal
Unda + Ignis : Refresh MP
Gelus + Telus : Convert 5% damages taken to HP
Flabra + Sulpor : Blocking +5%
Flabra + Telus : Parry +5%
Sulpor + Ignis : Critical damage +5%
Unda + Lux : Convert 5% damage deal to HP
Umra + Unda : Convert 5% damage deal to MP
Ignis + Flabra : Convert 5% damage deal to TP
Gelus + Lux : Enhance attack with stun
Unda + Telus : Enhance attack with silence