The word skill tree has just become the common term for a group of skills in the same category, regardless of whether or not progressing in them is in the form of a tree.
The Meta Mage idea is actually a pretty interesting one. Not as a class, but perhaps as a new type of metamagic spellbooks that can't be learned but can only be used in spellcrafting.
The Tank idea has been proposed by Larian as the Guardian, but those would make good suggestions for Guardian skills.
