The one killer feature I wish Syncthing had was untrusted peers like Resilio has. There has been an issue[0] open since 2014 but I guess there are still major challenges to this feature.
It would be great for me to be able to specify a certain node as encrypted and read-only like I can in Resilio. If a friend wanted to store his files on my machine and vice-versa, we could do so without being forced to give access to each others plaintext files. Likewise, if I wanted to spin up a VPS and host an untrusted node to help facilitate syncing but didn't want my unencrypted files to be sitting on the disk, I could do so easily.