I'd also love to see some some sort of magical cloning/doppelganger ability that improves as you level (additional clones). This would give the class a very different feel from existing tanks by allowing the mage tank to still have low HP, but when the character "dies," it's just a doppelganger that dies, and a replacement or the original steps in and takes its place with full HP. It would also give the devs a place introduce truly unique features for the class. As an example, a doppelganger that acts in your stead makes you immune to instant death attacks (because it's just the doppelganger that dies), which could be key in difficult boss combats.
I think it's a cool idea and I'd love to see it implemented.