Which skills are granted in which section of the passive trees is surprisingly important to how the skill and passive trees and unique items are designed. Making the skill unlock order variable would cause a complete audit of every node of every tree in the game. We would have to change, move or remove many nodes.
It's a cool idea that would have a much bigger impact than you might think.
It's not feasible to implement at this time if we did decide we wanted to do it.