I think it'll be 11 and 13 inch airs, with 15 and 17 inch pros. There's really no reason to buy the 13" pro anymore, unless you absolutely need the optical drive, which is very few people. Another possibilty is that they loose their names completely, and are now just called the 11, 13, 15, and 17" macbooks. Apple loves simplicity in naming, so I can definitely see them doing this.
The pros will keep their increased performance over the airs (quad-core and dedicated gpu), and their ability to be upgraded, while they will loose their optical drive, and gain the ssd (256 and 512 for 15", 512 for 17). They may go up in price initially due to the price of flash memory, but they will eventually get back down to where they are now. Oh, and I think the 15" will get a standard 1050p screen (with a more expensive anti-glare option), so that it beats the 13" air, unlike how it is now, where you have to upgrade it to get a better resolution.
They will be reasonably thinner, but not more so than the airs (maybe like .8 inches?), because they have the full power cpus and need a bigger battery to account for the more power-hungry parts.
Anyways, those are just my thoughts. If this happened, it would be very hard for me to decide between the 13" and 15", assuming they are within a reasonable price range of each other.
