This post from iOS developer Marc Palmer proposes a theory that makes a whole lot of sense to me: what if the Mac Pro and the iMac lines merge? Right now there are MacBooks and MacBook Pros. There is a Mac Pro, but nothing simply called Mac. It would seem consistent with Apple tradition to have a Mac and Mac Pro (both all-in-one machines), in addition to the MacBook and MacBook Pro.
I really like this idea. It feels right. It would simplify the product line significantly, and it would bring things back to the original 2x2 grid Steve Jobs introduced when he returned to Apple in the late 90s.