It should be possible to alter the case to support a slide cover, like for graphing calculators: slide it off, put in on the back when in use. slide it off, put it on the front when not in use. It should be resistant to puncture damage to the screen and prevent keypresses.
For LTE modules, as long as you have USB, you are able to connect one. If you want 5G, check the Qualcomm SDX55 and SDX65 (submm wave + 5G SA) on Telit. The easiest way is to get a Telit EVB + a Cinterion module.
If someone wants to turn this into a cell-phone product with a finished case (importantly not 3d printed with odor- or toxin-emitting plastic), audio, etcetera, I'd buy at least one, as long as it could be added to an MVNO plan.
For LTE modules, as long as you have USB, you are able to connect one. If you want 5G, check the Qualcomm SDX55 and SDX65 (submm wave + 5G SA) on Telit. The easiest way is to get a Telit EVB + a Cinterion module.
The only difficulty would be routing the audio pathways and maybe IMS registration for VoLTE, but you could use a Quectel EG25 like on the pinephone: the module can be controlled by AT commands and the existing work for audio routing could help you get started: check https://wiki.pine64.org/wiki/PinePhone_Pro and the audio part of https://xnux.eu/devices/pine64-pinephone.html like https://xnux.eu/devices/feature/audio-pp.html#toc-voice-call...