My prediction is that there are going to be just 2 computer lines, the MacBook and the MacBook Pro. They will come in the following sizes:
MacBook 12"
MacBook 14"
MacBook Pro 13"
MacBook Pro 15"
The MacBook will be for users who don't need much power and generally use their machines to write emails / papers, go on Facebook and browse the internet, watch videos, etc. Appropriate for the vast majority of computer users out there who can't yet use an iPad as their main machine, but don't need the power of the Pro. Sure this machine is expensive now, but so was the Air when it was first released. The MacBook is positioned to be the main computer for the average PC user.
Meanwhile, the Pro will be larger and more powerful to serve professionals who need to get more CPU-demanding work done on their machines - software engineers, web developers, graphic artists, video editors, etc. I believe the leaks we're seeing today about the "new Air" are indeed for the new MacBook Pro.
This lineup is clean, simple, and has a device suitable for everyone in the market for new portable Mac.