I run syncthing on about 10 devices across a few countries and networks. Love it. Love it a lot.
One thing I have been thinking is whether you could use your relay servers to connect SSH sessions without opening ports.
eg. NAS1 wants to connect to NAS2 - NAS1 announces wants to connect to NAS2. NAS2 checks if anyone wants to connect every couple of minutes, and when it sees someone wants to connect, the relay joins the two and they connect. Then run a script on NAS1 or whatever.
Obviously this is a high level idea, and I am sure there are great technical ways to achieve this securely etc.
I think this would be a killer feature.