Finally, Fedora 28 supports the RPi 3B+, with a functional 64-bit image! However, I had an issue with resizing the root (/) file system. THe instructions on their site did not work as documented, after running gparted on the image, the / volume was still small. I ended up running the following, as root, on the running system, and it worked fine:

$ lvextend -l +100%FREE -r /dev/fedora/root

I had of course alread run gparted which resized the PV pool, but I suspect you could do that too with 

$ pvresize /dev/mmcblk0p3