Last time they had a serious drop (during cata, but not nearly as serious), they did the free copy of D3 if you sub for a year.
I could totally see them announce a promotion like a free copy of the next expansion plus beta access with a 6 month sub.
On another note, my guess they will release the expansion in May to build up the population and hype for the new movie.
First part of the legendary quest in the next expansion. "In a far away land, there is a moving picture on a screen. Go and witness your history young orc (human, etc), and return to me with your ticket ID"