Actually, I feel there should be another two main branches that I miss a lot:
1- Thief branch: There should be anything related to ninjas, assassins, tricksters… and it would be a smart idea gathering the already existing classes that obey to that pattern, like rogue and corsair.
2- “Scientist” branch: There should be some of the most crafting intensify classes, like the classic blacksmith, alchemist (having it in the mage branch is kind of weird); some pet classes like genetist (that could create homunculus an so on) and mecha-riders (steampunk layout please! pshht pshht), and indirect damage output like engineer, putting automatic turrets and other devices (supportive and crowd control ones too!).
I may say there is no hurry. It just could be a nice idea to implement in the future, but please, keep that in mind.
I think a total of 6 main class branches should be enough to gather almost any kind of play-style any player could imagine.
Edit: Oh please, and bionic class too, with some kind of augmentations (drolls…)