There's a bit of convergence coming from the iPad Pro below the MacBook Pros at the moment.
Proper Photoshop getting announced by Adobe is big move and Microsoft already offer versions of Office for the iPad. The benchmarks for an A12X powered iPad Pro this year could very well challenge the 5w Intel CPUs in the current Retina MacBook which surely has to get an improved 3rd generation keyboard. Amber Lake CPUs may keep the line going for another year or two though.
What got me thinking was the current lack of a 15w Iris Graphics Intel CPU option on the Intel road map which would go into the non touch bar Macbook Pro and the 21.5" iMac.
What if Apple were to merge the line with the MacBook Air?
What about a 13" MacBook (with or without Retina)? The precedent is set already with the Retina MacBook 12" using HD615 graphics.
It would just be a rebranding exercise as Apple would be redefining the CPU that went into the previous non touch bar MacBook Pros. Apple could potentially go for a cheaper CPU, and possibly live with a HD (IPS?) screen rather than Retina and save a couple hundred dollars.
Apple could then go with the following options for ports:
a. Use 2 Thunderbolt 3 ports from the non touch bar Pro, minimal case retooling required.
b. Use 4 Thunderbolt 3 ports but the right most 2 are half speed (as with the 2017 touch bar MacBook Pro)
c. Use 4 USB-C ports (losing the crucial Thunderbolt option)
d. Use 2 USB-C ports (the very cheap out option because this is a MacBook Line)
Yes, there will be people recoiling in horror as Apple get rid of Thunderbolt 2 ports but also every other popular port plus Magsafe.
The Lineup then becomes:
12" Retina Macbook (which should really get 2 USB-C at the very least)
13" Macbook
13" Macbook Pro
15" Macbook Pro
And with iPad Pro catching up the Retina MacBook from below.
As a footnote, the Touchbar powered MacBook Pros have CPUs that have 16 PCIe lanes, not 12 from the 2016 and 2017 models. Therefore all 4 ports should be capable of full speed. There used to be an Apple tech note about this but it was edited in April of 2018 and references to half speed are gone.