The 4th Tank.

[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