Yeah, it's strange that a laptop style portable console never received any focus, considering that the PSOne had an official screen accessory, and the GameCube was built to take a battery pack and screen.
Sony may be best testing the waters with an iteration of an already successful platform, and, come 2019, release a PS4Micro in the vein of PSTV/PSVitaTV (only, not shit) :
-- No HDD or ODD --
Just a small SD card for local storage, but plenty of USB 3 type C ports.
-- Improve PS4 interoperability part 1 --
Let one PS4 access another via Remote Play.
You can sort of do this at the moment via Share Play, but only sort of, meaning you can't login to someone else's PS4 and access your own, primary PS4, whereas you can do so with your Vita/smartphone.
-- Improve PS4 interoperability part 2 --
If you place a game disk in your primary PS4, you may login on a secondary PS4, establish a link with your primary console, and download game data to the secondary console. As long as both consoles are connected to PSN, the primary console has the disk inserted, and the primary console is going unused or being used by another user, then the secondary console can play the disk based game.
-- Release a laptop/tablet style enclosure --
Personally, I'd prefer a laptop style to protect the screen and more easily accommodate a decently sized HDD. Include dual cameras at the top and some wireless streaming solution for PSVR, and you have a compelling accessory IMO.
Then, transition the PS4Pro to a mass market price, and make it a solid hub. Maybe take the opportunity to include a UHD BR drive. Definitely improve its streaming capabilities, up to 4K60 streaming and recording, simultaneously.
At this point, PS+ will no longer provide PS3 and PSVita games, so use the revenue and time before the PS5 to evolve the network:
-- Turn access to the network into a two tier system: PS+ and PS+Now. In other words, rebrand PSNow, and functionally alter it, so a subscription to PSNow will also get you all of the features of PS+.
-- Make PS+Now more functional than it is at present. It needs to operate as though you're renting your own PS3/4/4Pro, and allow you to play any content that you've bought, as well as its current Netflix-style library.
-- Bring back a refined version of PS Home, and make it a natural interface for all system functions: launching games, going to the store, meeting up with friends etc. If you're subscribed to PS+Now, you can enter Home and only suspend your current game.
That lays the groundwork for a PlayStation in every room, and on the go. And it does so without betting the farm on a new platform.