Just things I'm thinking about off the top of my head. There are other uses for it, obviously (it'd make a fine broker for Proxmox/Ceph management I suppose).
Yes, I had thought rPi units would make an interesting Ceph cluster, using each as an OSD host for one or two drives. Major downfalls being only one ethernet (no separate management interface) and the HDD/OSD being attached over USB. Might not be fast, but would be darned cheap.