First I would like to introduce the why Samurai would be the perfect fit as the next tank class. I wont try to make this too long and if you have any suggestions or ideas please feel free to post.
Current Tanks (I won't go into too much details about the current classes as you are all probably familiar with how the two function).
Paladin - Mitigation Tank with average AoE skills and generous support skills (Cover, Tempered Will, StoneSkin (Xclass), Cure (Xclass), Protect (Xclass) etc).
Warrior - More active/micromanaged based Tank with exceptional AoE, large health pool, good damage dealing potential for a tank. Warriors can be more prone to spike damage unless you manage defensive cooldowns well and have good timing.
Looking at two current tank classes we have to think how can a third tank fill the roster without overshadowing the current tank(s) in the game? Easily, by implementing an evasion/counter-attack tank such as the Samurai.