Eurhetemec seems to grasp the idea I was suggesting, perhaps I should clarify further:
The moment you take this currently undefined "tanking talent" your mastery bonus will switch to a tanking version. Which bonus is granted still depends on your MAIN tree.
Your point Mecer, that there is no true tanking talent is debatable, but lets not forget the whole talent tree will be redisigned.

My whole point is that each tree should/could get a tanking-switch. Picking more of these switches won't mean more mastery bonusses, only add the talent bonus ALSO found on the switch talents.

Example:
Going 5/8/5 would mean you get the frost mastery bonus.
If one or more of these talents is the tanking-switch talent, you get the frost tanking mastery bonus becausethat's your main tree.

Ok, just to be sure none misunderstand, this could be an example of tanking-switch talents:
*Blood: 5% armor, also changing mastery to tanking version (of your main tree). 1/1
*Frost: 3% parry, also changing mastery to tanking version (of your main tree). 1/1
*Unholy: 2% magic absorption, also changing mastery to tanking version (of your main tree). 1/1