I think it could be a good opportunity to deter players from freezing accounts on top of attracting newcomers. It would be a nice, temporary trial for those who haven't experienced level 20+ content.
It just isn't financially justifiable for me to continue playing WoW throughout content droughts. IMO, dropping the sub fee for expansion gaps would be worth the revenue loss in the long run.
EDIT: Valid points. My opinion has shifted but I the initial expansion cost on top of the sub-fee is ridiculous. Why can't expansion release months be sub-less?