Thanks for the feedback, my concern is about real offline mode where some tiles, terrain and images are on the device to allow to work without internet connexion (or with low bandwith).
Cloud distributed data offer by Cesium ion or on premise solution as also you can propose require a network access which is not so bad in bandwith, but it is not always the case in real life for some scenario.
To support a minimised set of use cases, it can be useful to have onboard some data to optimize available 4G bandwith if any ( will be an another story with 5G).
In between, I have succeed to manage offline some of my terrain dataset by doing a small code update to apply a gunzip on the read contents for url starting with file:// ( in http, this is done on the fly by the web client).
After some tests, I will potentially share it as pull request it there is an interest for others.