I wanted to have iCloud Airport Time Capsule Server for a long time. All iOS Backup, super fast along with fast restoring from local. Currently restoring an iOS devices takes a long time and often get stuck for whatever strange reason. Local caching of iOS MacOS Update. Along with caching of App Store update.
I think all the above are already available in macOS Server iCloud Caching option. Except no one wants to spend $799 for it.
I do think this devices should work along side with iCloud and not on its own though. It is hard to avoid HDD Failure, File Corruption, Bit Flip etc. Cloud Still gets you much better Data Safety. ( Someone else taking care of it )
Some Airport/Time Machine solution where I can set my iDevices to only sync locally to that device.