I feel that Blue Mage could make a very good Tanking class in this game as it is fairly durable in most titles, has a wide variety of applications and can wield a variety of gear. The quest chain could involve studying a monster's move by battling it and acquiring it at the quest end, There is a good number of previous blue magic spells that would work for tanking as well, such as:
Mighty Guard - Defence Buff (Possibly for the whole party)
White Wind - Low Grade AOE Heal (Generates enmity)
Goblin Punch - Basic Skill (Generates enmity)
Aqua Breath - GCD AOE skill (Generates enmity and lowers enemy strength or defence)
Vampire - Single target skill (Drains HP, generates enmity)
Auto-Life - Mass wall skill, could work similar to Holmgang and Hallowed Ground but have a large cooldown period
There are countless others that could be implemented (1000 needles, Magic Hammer, Quake etc) to make it a viable tank class but those are just some examples on how I feel it could work.