I think that once they get the price of the Retina Macbook Pro to $1999, that's about as low as it's going to go for a while.
In 2006 a base Intel Macbook Pro with dedicated GPU was $1999.
In 2008 a base Unibody Macbook Pro with dedicated GPU was $1999.
Only recently has that bumped down to $1799 with Macbook Pro.
Apple very rarely makes significant price changes to product lines. One of the most noteworthy was the Macbook Air, but that was a complete redesign and refocus of the product.
Of course the specs will increase to keep it in that price range. Maybe a 512GB standard SSD in a year or two, with spec updates to CPU and GPU that follow the trends of the Macbook Pro line over the last several years.
I think that a 13" Retina Macbook Pro will eventually fill the $1500-$2000 slot, with the Macbook Air below it.
Exactly, 13" retina will likely start at $1500. Time will tell whether the 13" MBA or the 13" retina will sell the most. I know most college kid's parents will dish out for a 13" MBP at $1200...but at $1500+, lets see or else they'll have to drop to a MBA if they can't afford the retina