Sry but have you watched a gameplay video of D:OS?
In D.OS the Hereos are Source Hunters that means they use the power from the Source (i dont know exactly the power of the Source since in DD and D2 the Source is just a healing-gem prodicing thing).
But in DD and D2 there are no special powers needed to be a magician so it would ot fit into Rivallon if somone would have to use a certain substance.
Combined Casting: You know that D:OS is TB and that there is a lot of combination between the schools of magic. Like using lightning on water surfacestuns all enemys and a lot more.
The chance that spells would be totally annihilated or turned into the opposite is against the strategy of D:OS. If I have a certain strategy how to kill the enemy and now instead of freezing and stun the enemy it would just a lava surface?? I mean that would mean planning something will be useless.
