PS: Someone should probably go through the other suggestions and evaluate which are more and which are less realistic…
stronger battery (even if it means a phone with a hump)
I know a guy
alternative E-reader display
Sure, but complex and thus expensive. Dont count on it.
Solar Panel on the backside and/or integrated in the screen
Sure, will charge ~2% per hour, you decide whether this is enough. (in screen: no.)
programmable remote control
emergency button for elderly people
Sure, but not improvement from current systems
additional sensors: barometer, temperature, sonar, radiation
a waterproof built-in case/back with a built in flip-cover
Maybe from company that does this on regular basis
Charger for a spare battery.
Alternative hardware components that allow software freedom. E.g. a
i.MX6, OMAP4/5 or Allwinner chipset to allow the installation of Replicant
Very very complex.
hardware switches for phone service, camera & microphone2
Will need replacements on modules, so phone service no, camera maybe, mic yes
built in projector
No current pico projector parts available for reasonable price, so not now
USB-C port, sure. Functionality of both will depend on what is available on that connector
Hope this helps