MacBook different models should be designed around each family of intel processors.
They don't use 45W (Haswell) in the rMBP13, it would probably need more thickness for a better cooling system and to accommodate a larger battery.
They don't use 35W (Haswell) in the 15" for an even more thin design because they already use it in the 13" and with the volume gained allowing more battery they can afford fitting in higher TDP chips with higher performance.
Etc.
Growing in size with chip's TDP allow to design a balanced line of laptop: equivalent design, similar battery life, gradual performance. But the Air was the exception using at any size same TDP chips with compromises to keep small sizes. With Broadwell then SkyLake, and future 10 nm lines, previous performances and improved ones will be accessible on lower TDP chips. The direction taken is to consume less power, not to only offer more performances; new lines of intel chips will probably offer lower TDP without concession on performances, with something like 3 families (sub-15W>10W, sub-30W>20W, sub-40W>30W) + Core M sub-5W.
Convergence of the two Apple laptop lines is for me the most logical future, offering more consistency from the bottom to the top, and an overall simplification of the offer, with a decision for consumer essentially on size; performances logically follows.
In that perspective, the unique/converged line of Apple ultrabooks would of course have retina screens, and for consistency the pixel density wouldn't vary much between models (contrary to the current situation of the Air having a quite higher pixel density since the introduction of the 11.6" model that had to offer enough screen estate i.e. had to have a large enough resolution for actual capability on a desktop OS; the new 11.6" replacing directly the previous and first 13" MBA model, the maintained 13" new model had to gain a larger resolution too, for differentiation and consistency inside the Air line, but de facto offered the same resolution as the MBP15).
This would give something like:
. 12.2" built around sub-15W chips at 2x 1366x768 (~255 ppi)
. 13.3" built around sub-30W chips at 2x 1440x900 (~255 ppi)
. 15.4" built around sub-40W chips at 2x 1680x1050 (~255 ppi)
All offering a thinner profile than current rMBP, approaching MBA lightness and design, without performance loss.
Adressing the low-end (price-wise) will be with iOS, wether they introduce new form factors like an iOS touch laptop or with larger tablets.