I'm not the biggest fan of stance dancing. At least not the way it is now with swapping offense for defense. Healers losing Cleric was godsend.
Now, the way you describe where you are trading one offense for another, I think would flow a lot better. The same idea could work defensively as well giving tanks a true parry stance. However, I am not sure how this would reduce/prevent button bloat. Actually sounds like more would be added to pull this off.
I really like what they've done with abilities like Stone Mastery and Broil Mastery. I think they could implement this with current abilities and upgrade them in the same fashion. As an example DRKs Carve and Spit can upgrade from a three-fold attack to a five-fold attack at level 72. This would allow them to add two animations to the existing ability without adding an additional ability to put on your hotbar.