A little brain game, and some working of the imagination bone.
Which core trait for your profession seems to be the "elite" or essential trait for most of its meta (or best it can do) builds?
I would say that trickery is the thief ELITE spec.
Now, after you say what it is, imagine you could not take it when you chose a real ELITE spec.
To justify that change, how could the traitline be buffed to be a true ELITE spec?
Soon™ ( ͡° ͜ʖ ͡°)