I have a hard time seeing blue mage as a tank. At best I can see it as a hybrid DPS like Summoner. Which, don't get me wrong, would be pretty cool.

I'm kinda hoping we get a modernized version of mystic knight for the next tank class. Maybe make it a dual sword wielding dervish style combined with some time mage slows and haste abilities as it's tank gimmick. Litterally slowing the enemies DPS while enchamtog his attacks with various magic and status ailments.