I would really like to see this too, I even commented on Yoshi-P's comments about it
here.
I really don't see why it is "stressful" from an adventurer's point of view. He kept insisting on how the main quests are going to be battles because we have to see it from the adventurer's point of view, not the classes by themselves. Then, for mixed dungeons, he goes back on his words saying things from the class's point of view. I don't get that, we can switch to battle class and
vice versa anytime (except during battles).
Mixed dungeons would be fun and not that difficult to balance since adventurers can change class in different parts of it. It would simply require to have the adventurers level up a Discipline of the Land or Hand class in addition to their battle class (wait, isn't that the purpose of the armoury system?! What a coincidence!).