I don't see much more that can be done, besides thinning, increase and use faster memory and SSD sizes.
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A convertible tablet for one. Put the slimming of the cases and the thinning of the bezels to use (except the wow effect) and make a usable convertible, that you can hold in your hand for reading and put down for serious writing with an OS that sports a full fledged file system, the possibility to use whatever apps you want and trackpad/mouse support in Laptop mode.
Just make the tablet part multitouch/touchscreen and adapt a few essential passive (consumption) Apps like Preview, mail, media players etc. in a portrait tablet mode et voilà. It should be really easy to do, seeing as iOS and MacOS are derivatives of the same source. Just let devs adapt the interfaces of their Apps to the new mode and you'll have an ecosystem in no time.
Sadly Apple is too stuck in selling their iOS stuff (iPad Pro lol) for this to ever happen to a single device (You need to have all their devices with dongles etc.) , while Microsoft manages to pull it off and HP, Lenovo etc. delivering the necessary hardware with their latest kabylake iterations. These will get even better the next year and the years to come.