A MacBook is different to a MacBook Pro. Gripe as we may about the MBP but that "Pro" bit enables creatives.
The fact is most people don't realy create anything. They consume. They type. They browse. They store.
For them, a Macbook would be perfect - I have a friend who just picked up an MBP and honestly, I have no idea why. They have zero need for it. But a 13" Macbook would have been perfect.
Not same specs, it doesn't need to fly like that. The form factor, I'd argue they may just slim it down like the 12" Macbook, not sure. I think they would basically either enlarge the 12" and make it a bit better, or do a cross between the MBA and the 13" MBP.
Whatever they do, it makes sense. There's a middle ground which isn't being catered to properly. Also the Macbook keyboard sucks.