Next Tank Job?

[Justuas]

Considering that Samurai has and still has seigan along with third eye

Third eye doesn't absorb magic, and also the most problematic attacks are aoe which ignore third eye. Edit. Third eye doesn't absorb aoe magic.

access to heavy armor

Actually no. Only war, pld and drk have access to non-job specific heavy armor.

with -% damage taken on it

Just like every other job have access to -dt gear. However, only pld and run have the best -dt gear.

counter attacks

Can't counter magic or aoe attacks.

samurai was and still is a tank in FF11

Oh please, find me a tank samurai.