If SE opted for the specialization route, multiple classes could have the same trees. CON and THM could both have a White Mage tree that vary slightly from one another. For example, THM/WHM might get stronger light-based nukes and HoTs while CON/WHM would get stronger direct heals and buffs.

If they went with the advanced class system, which, if I understand it correctly, suggests that we would need to level specific classes up to unlock them, I see CON a much more likely prerequisite for WHM, assuming there is just one. Thaumturgy is pretty dark.