Clearly, the easiest way to reduce the number of spells on the action bar is to remove some of them. Either fold separate attacks/spells into a smaller amount, combining them or just flat out removing ones that see less use. We saw some of this moving into Mists of Pandaria (one example that springs to mind is the folding of Deep Wounds and Rend into one ability) and we could always see more. Of course, the concern is always that you're getting rid of someone's favorites in the process. At any rate, we have only the idea that there are plans out there, and that they will cause some crying.
Keep an eye out, folks.