Say what you will about Warhammer Online, it had a great many ideas for breathing life into class specs and making them fun to play in PvP. I haven't played it in quite a while because of the shrinking population, nor have I played world of warcraft for longer still, but out of curiosity I decided to check up on how the game looked after all these new changes in mists. One thing in particular I noticed was the Battle Healer glyph that functions similarly to how melee healing worked in Warhammer Online. One spec for Warrior Priests (albeit not the most notorious, but certainly the most fun) focused on healing allies with lifetaps, and was a very risk/reward driven playstyle. Before that, I had never played a healer nor had any interest in doing so, as they always looked boring. Melee healing changed that.

With that premise setting the stage, I'm not sure why Blizzard hasn't attempted to liven up PvP for tank specced characters with the ability to focus on mitigating enemy damage and providing support. To use Warhammer as an example again, healers didn't really have many tools to prevent damage, they were mostly in the business of healing after it had been done. Tanks on the other hand had a great wealth of ways to mitigate incoming damage; taunt reduced the target's damage dealt to everyone but the tank who taunted them until they hit them 3 times (which could have taken a while if they had a good deal of avoidance), each tank had an ability called Guard which transferred 50% of incoming damage to the tank, as well as plenty of crowd control and damage debuffs to round out their kit.

So why is it that tanks are still a PvE only spec? 

Yeah, interesting. SWTOR had this, too, and I think it worked well. Good damage mitigation is really a solid advantage to bring to a PVP encounter, and it makes the tank feel less useless. Same with giving the tank various ways to annoy and delay the enemies.

I can't see a good reason to not implement it.

They did during the 4.0 era and the amount of bachlash from players forced a nerf.

Used to be able to take on anything, slow as shit to kill someone but killed them in the end.

Also funny as hell holding points indefinably as a pally tank.

Blizzard should buy Wargaming and then make it World of Tankcraft. Just imagine how many players the game will get!

I am not sure what the difference was in warhammer.  But in WOW, tanks can use all of their survival abilitie to mitgate damage.  I pop stuff like survival instincts all the time as a druid.

If you are just talking about mitigating damage for your teammates..  that is kinda how a few of the healing classes work. Disc priests in particular.

Should tanks be able to ´taunt´ enemy dps to attack them?  In a way, that is just a form of cc.  If I force you to attack me for 6 seconds and you do only 20% damage to me..  it works out the same as if i just cc´d you for 5 seconds.

It makes pvp slightly different, but I don´t think it makes it any more fun.