We already have specs, we're adventurers, and we can play any class or role we want. the ultimate tree system.

In all seriousness though, they experimented with this very idea, Summoner and Scholar were their attempt at a specialisation within a class and it turned out to be a terrible decision that they've chosen never to do again, instead introducing a single role for each new weapon type instead.

The game just isn't designed around multi tiered classes. forcing change to shoehorn it in would cause a lot of problems too.