Would it be possible to include an additional sync via Bluetooth in NC in addition to the normal network?
If there is no internet connection, it would still be possible to synchronise between clients that are within range of BT.
Possible application: ship at sea without internet. I can write contacts or texts on the smartphone, can synchronise when I am on the PC or tablet in the cabin and continue editing there.
Second possibility:
I travel a lot by car and would welcome it if I could synchronise all devices within the vehicle via BT and then only one device communicates with the internet and the remote server. This would significantly reduce my data traffic via mobile. My thought would be to prioritise the sync options. First check if BT devices are in range and sync them. Then, if possible, sync via the internet with the remote server.
I use my smartphone, tablet and notebook on the road and write a lot. I can use any of the devices for this thanks to NC. But unfortunately it only makes sense if each device can communicate with the remote server. If all the devices could also synchronise with each other via BT, it would help me a lot. I can imagine that many travellers have this problem. I run several clients and have to synchronise each one with the remote server via the internet. This could perhaps be eliminated by syncing between the clients via BT.
Possible is almost anything. The question would be how easy it could be implemented, if sombody has time to implement it. And last but not least it’s about demand and priorities.
About the contacts. Since Nextcloud completly relys on third party CardDAV clients and only provides a CardDAV server, I don’t asume that they even remotley think about developing something as complex as this for their official clients.
Regarding the files. You could use some third party software for synchronizing the files between your laptop and phone via Bluetooth and then copy them to the Netxcloud Sync folder afterwards to avoid sync conflicts 
You could use USB or WiFi teathering on your phone to connect the other devices to the internet.
thanks for your reply.
I didn’t take CardDav in account.
But to take third party software for synchronizing via BT is a good idea. Im going to try that.
Tethering is what I use now. It works of course, but it does not reduce my datatraffic
Your best bet is KDEConnect. You can simply tell it to save files to your Nextcloud folders, which will then sync to your server.
It is supported on all major desktop and mobile platforms. Fully open source. Works well as a multi-function device that doubles as a remote control and allows you to sync sync files, SMS texts and notifications between your devices over wifi and bluetooth.
nice, thank you. I’ll try to get it to work