I understand that it is difficult and perhaps it should have been raised at another point in development. But if they ever want to do it, they could use a system similar to d&d: there is a total level of classes, so each level in one class implies one less level in another class, this implies that you will not get high skills in any class, which justifies exploiting the combination of low-level skills. If there is still a particular skill that breaks the balance, it will be prohibited from acquiring it in multiclass
I agree that it can be made, but I don't think that is a good move at this stage, because not all classes are implemented in the game, thus they are not fully balanced at this point. If we go on the road of multiclass before we have balanced our current classes and specs I think that would break many of the classes.
After the game is published and classes are optimized, that can be made as a separate mode/server/realm.
And also since you mentioned warrior mage, we have some pretty cool ideas for the classes and they won't be generic, you can count on that, think about battle mages with swords, exorcist priests with cross-bows and interesting stuff like that š