Now SAM Tank dreams have been crushed... I would like to see either a Warhammer Tank or a Dual Wielding Tank.
I feel a Gunsword job belongs to melee DPS in maiming gear.

Warhammer Tank could be like what Eric mentioned which sounds really cool! Or I could envision a holy crusader that would be related to PLD but use it's holy magic more offensively.
Maybe a Magical Tank in same veins as DRK ie utilizing MP a lot more then the others. Would use it's Hammer as a Magical catalyst and create self barriers! (That self barrier tank some of us have been wanting)

Dual Wielding I could see BLU doing quite well, would be a fast paced tank in the same veins as MNK/NIN speed. Or if they want to save BLU they could use Mystic Knight with a Dual Wielding take, I could envision it's AF being heavy arabian themed armor.