I haven't read every single comment on this topic, but here's my idea of Apple's future lineup (based on some vague rumors / my ideas):
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I quite like how this turned out.
Pretty simple thinking really; MacBook in 12" and 14" sizes which replace the 11" and 13" models (MBA & MBP), 16" MacBook Pro which replaces the 15" MBP.
The screen sizes sound confusing at first, but in the end you get more screen real estate for every model in a similar sized housing (I modelled the picture above to the "Compare Mac Models" picture from the Apple site with 12, 13 and 15" sizes).
Personally, I really like the 12" MacBook which is why I wanted all models to look like it. Additionally I think the current 13" MBP has to much overlap with the 13" MBA which is why I'd replace it with one 14" MacBook instead.
Now comes the 'future' part; processing power.
Basically, the current 12" MacBook doesn't perform as well as the Airs. So for the future, it should be comparable to 13" MBA - 13" MBP - 15" MBP power wise, ranging from the smallest to biggest screened model as pictured above.
Maybe the Pro gets fans and gets a little thicker, don't know about exact processors. Battery life should be around 9-12 hours. 14" MacBook should have at least two Thunderbolt 3 (USB-C) ports, 16" Pro should have all its Pro ports.
Oh, and they all come in Space Grey, Silver, Gold and Rose Gold