UPNP can be handled by existing Linux UPNP software.
There is a FOSS VPN that can do point to point VPN routing, but I'm forgetting the name.
FUSE works ona a surprisingly large number of operating systems now (Linux, OSX, Android-fied Linux, Windows), so building a FUSE file system can be done.
And as for data sync and replication? CEPH can probably handle this.
The software exists to do this, no one has put it together in an easy to use package. This is why I funded their effort, I want this to come to pass.
Indeed, I wasn't suggesting that any one of those things was technically infeasible, but that "putting it together" is a very large undertaking, because there are a lot of things they're trying to put together.
The software exists to do this, no one has put it together in an easy to use package. This is why I funded their effort, I want this to come to pass.