As strange as it sounds, I think MMO's could do with stealing mechanics from MOBA's to make their tanks more interesting. Guys like DOTA's
Tidehunter can show up out and instantly turn a fight around with his ultimate.
Earthshaker brings a unique mechanic in the form of creating an impassable wall on the battlefield. LoL's
Alistar can forcefully relocate enemies away from his team.
Why don't we have cool, interesting, tanking mechanics like that in MMO's? I mean, I know we're probably never going to get Dark Souls combat in an MMO, but there's been a very noticable lack of "new" mechanics that have been added to the genre in a long time.
Maybe we should also be moving away from the tank being "That dude with the armour" too. Thematically there's nothing wrong with the idea of, say... A Mage tanking by creating magical barriers for himself and what not. Gandalf, the Wizardiest Wizard that has ever Wizarded, tanked a Balrog in the LotR movies afterall. A Battlemage tank with a sword in one hand and a staff in the other could have a lot of appeal to it.
Or perhaps a Matrix Dodging Rogue tank? Or what about a Hunter with a Pet that can tank? Guys in armour only fulfill one class fantasy, by expanding on the number of fantasies that are capable of tanking we open up the role to players with different playstyle and asthetic tastes.