For me I would go for:
-e-ink display: having boarding pass on the airport, any e-ticket would be nice
-LoRa module: as something I have never tried but I am curious
-kayboard or retro gaming pad: for easier typing or having fun with emulated NES, SNES, GBA (and other platforms) games
-GPIO pins: for whatever I did not think about
I started writing a native Meshtastic client app for Sailfish, but development stalled after someone stole the repeater node on the hill. 
You can buy that already…
Depending on how easy TOH is to pop out, having the card hidden inside would be way cooler, though.
Well, the camera would make it a popup from the phone bottom, or rather a popdown. I can see the small catastrophe in shop already. 
And as I need room for min. 5 cards (RFID protected), it vill be thick as a brick. 
Card holder needs no pins or digital/electronic connection to the phone. I really hope that we get a “blueprint” 3D-File for the back cover even if the TOH goal isn’t reached. Creating a card holder back would be easy then.
What I would find interesting about a battery ToH would be the use of current batteries if the internal battery were to become difficult to obtain later on.
GPS is must have, but extra battery should be available too
Why exactly you need another GPS in the TOH?
I mean that the device should generally have GPS, if not as standard, then at least the option to add it. So there is no need for another one.
Also, to clarify, in addition to the earlier noted capabilities in the Tech Specs section at the product page the supported sensors include gyroscope, compass, accelerometer, ambient light, proximity and the usual combo of GPS/Galileo/GLONASS/BEIDOU for positioning.
Didn’t they already promise to release the STL-files for back cover early next year. Probably when they have locked down the frame dimensions and internal layout.
This could be awesome
Sounds a lot more like a case than a TOH.
I wouldn’t mind paying for a TOH which enables wireless charging
Camera TOH - camera lens mount + handle (includes battery)
Lens TOH - camera lens mount
Travel/Hiking TOH - extra battery + flashlight, (E-Ink would be awesome for small map), extra cover
Book TOH - pure maximum size e-ink with side buttons for back & forwards
Keyboard TOH - flip, slip or snap out keyboard
Hub TOH - cooling, USB ports, audio jack, display ports (HDMI/DP) to make your smartphone more portable computer than just a phone.
Radio TOH - antennaes for hacking the waves
Shoulder/Strap TOH - never drop your phone if it’s attached with a strap.
Bumper TOH - safe to drop, for people who have a tendency to drop things
Minimal TOH - for different colors to fit your mood or aesthetics!
That would be the easiest way to get contactless payments on sfos actually 
Don’t forget a global shutter! As opposed to a rolling shutter. It increases cost and mostly benefits video capture but the difference is night and day when quality matters.
Ultra telephoto camera module → a flat periscope style camera which runs the full length of the phone → high magnification with big reach → the phone doesn’t have one in the base design so adding an optional one makes sense for those who need it.
I would get much more use out of my current phone if it allowed me to read the names and numbers of distant ships and other vehicles. I would have a difficult time deciding between a thermal and telephoto camera though. Both are useful but not all the time.
If there is a camera module, would be great to do this and make it M43 compatible. There are tons of great M43 lenses and the sensors are cheap and decent.
Plus the Jolla with an Olympus 75mm 1.8 would be a sexy, sexy beast.