I already know that you are going to say that I should use the mobile apps for backing up mobile devices, but no, this is a different story altogether. This is a feature request to enable a NextCloud app that enables a device to connect to the NextCloud’s USB port, and when connected, uses adb backup (and to restore, adb restore) to backup and restore your phone without needing root. ADB backups have a prompt before the backup gets initiated but can be bypassed in a second by using adb shell input to simulate a keyboard pressing down, right, then enter, then optionally home. After that, the ADB backup can be browsed with a simple script that turns it into a TAR file, and optionally process it into a browsable rootfs via Andriller. ADB backups are currently deprecated and apps using Android Snow Cone’s API level on Android Snow Cone or above will only backup to Google One, however using on earlier version of Android such as Red Velvet Cake or Queen Cake will backup the apps, as well as using an app targeting a version below Snowcone as well. adb shell ls in conjunction with adb pull can allow /sdcard to be backed up with the lack of the app, if the user does not want to install the app for any reason, and the backup functionalities can be made wireless (but local network) if adb tcpip is used, however this does not persist after reboot, and Red Velvet Cake and higher provide a wireless ADB function. For iOS devices, libimobiledevice will be used to be able to use iTunes backups via the idevicebackup functionality, and be untethered from the USB cable via WiFi sync but still requiring use of the home network, but persists after reboot and does not pose security risks when connected to another network. libimobiledevice also has ways to backup other data via the iTunes filesystem (not as much data as the actual backup) like photos, settings, and apps that support itunes file sharing but not files app sharing. There are also tools on github to explore iTunes backups as if they were zip files. There is also a final thing I would like to mention, and that is to consider ALEAPP and iLEAPP for parsing backups into something easy to read for a non-techie.
Nice feature indeed. However an app like that is involving 3rd party libraries and depend on the individual users local middleware and host setup, A LOT! As such it would be nice if someone made an app, however I can fully understand why this would never be a Nextcloud core thing.
That would be a very niche use case. If at all, this should work via network. If it needs a USB connection you could as well just do this on a PC, with any existing tools, and then synchronize the backup file via desktop client.
Oh! Well, you should probably have started a new paragraph with this sentence, not only for the sake readability, but also because with this sentence your story takes quite a significant turn 
So your proposal would only work with older Android versions, which would make it even more of a niche solution…!?
Sorry, but this is very hard to read. You don’t make any paragraphs and you jump back and forth between things. Also, I’m not always sure, whether a certain sentence describes an existing feature in Android, or if some of them would still have to be hacked together…
Anyways, I think, as @Kerasit said, this has way too many moving parts, and I can’t imagine something like this ever coming as an official app. However, as a community project, maybe on XDA (is this still a thing?) a lot is possible.
And btw here’s a tip, if you decide to start this project by yourself: I wouldn’t focus too much on Nextrcloud integration, at least not at the beginning, because that would probably be relatively easy to put on top of the rest, once you’ve got that rest to backup the zillion different android versions and devices out there reliably. Also, why limit the use of something like that to Nextcloud…? 