I personally think each of the tanks should have styles of encounters they are best suited for and styles they are not.
I disagree, only because Tanks are hard enough to find. Who wants to play a class which changes drastically the moment you get in a group? Who's abilities are worthless when alone but suddenly they matter while your in a group? Non-healers and Non-tank classes are the KINGS of MMORPG's. They out-populate the others by a large margin. To then fracture the Tank classes into suited area's would then create an even larger hardship.
I don't think you have a bad idea, it sounds kind of fun-like you can be specialized in a certain type of fighting. AOE fighting or Single Target fighting. You could even have a different way of fighting, avoidance instead of blocking/parrying or Taking big hits but not parrying/dodging/blocking anything due to another mechanic which makes you thick or regen health. The problem is the popularity of Tanks is low, just like Healers popularity is low. Unless Tanks suddenly become 'fun' to play for your average joe, like DPS classes are, then Tanks won't survive something like what you are suggesting.
I will also say, there will be an endgame and a content drought as there usually is. When this happens do you have faith the producers of this game will even out the content so each of those types of tanks will be equally useful? Or due to unforseen player advancement, the good ol' meta breakers, the only tank to roll is tank X because of tank X's natural ability works in all situations due to X Y factors. My money is on the players, they always mess things up :)
And again, I actually LOVE your idea. I like feeling 'unique' when I roll my class and I can pick and choose the exact situations I can shine in. It can be so worth it, but on the other hand you may be very punished. Interesting double edged sword.
-Todd
Well not all DPS classes are created equally though, Some are better at sustained/burst/AE etc, I was trying to find a way to superimpose that image onto the tanking classes. Though you bring up some very fine points.
Though to be clear I think every tank should be able to tank an encounter, but I don't think "all tanks are equal on everything" works either, due to the fact the classes work in 3 different ways.