Something like this trifold phone and tablet makes sense if they put a screen on one of the thirds so you can have a pocketable phone that can expand into a tablet.
http://www.eyebeam.org/reblog?page=0,127
So if you take a galaxy note 5 which is 6 inches tall and 3 inches wide and then do a tri fold display you can get something like a 10" 4x3 tablet since 6 inches by 8 inches is literally a 10" diagnoal 4x3 tablet
Now currently the ipad air is a 6.1 mm device, and the galaxy note 5 is a 7.6 mm device.
So depending on how thin they can make a trifold phone we are talking 1 inch thick with a 8 mm thickness for each third or a 3/4 of an inch with a 6mm thick third. Most slider phones are 5/8ths to 3/4ths of an inch thick so we are still talking about a pocketable or small purse device but now you can get something that is like an ipad air in an easy pocket.
Oh since the tech that apple pencil uses, n-trig, and wacom es uses puts much of the sensors in the pen you can also have an active stylus for that 10" 4x3 tablet that folds into a galaxy note size but slider or slightly thicker thickness. Now a stylus is a second thing to carry but we are talking a lot of functionality in something you can easily keep in a male's pocket in not skin tight genes or a small women's purse.
The real question is the material science up to task, not necessarily the screens but the rest of the material that will give the device form and structure some of the time and no structure some of the time.
Now such a device would actually have amazing battery life in phone mode but crappy battery in tablet mode due to the extra thickness allowing more battery per device.
You could also implement a eink display for another 1/3rd of the screen opposite of the tablet and phone side.