On a pi, specifically?
Mine is currently running Mailrise and serving as a qdevice for Proxmox. It used to run nginx as a reverse proxy, but I moved that to a different machine. I had a second pi specifically for sharing USB devices over the network, but I wasn’t using it very much so it’s currently not in use.
If you’re looking for general ideas, I think a pi would make a good appliance for ddclient, Homepage/Dashy, an SSH/VPN jumpbox, UPS monitoring, or a notification platform. Basically, any set-and-forgot service that you want to keep running 24/7.
I mostly learn from mistakes, and since homelabs are all about learning, there are bound to be mistakes.
I’ve borked my network multiple times, broken VMs, and redesigned things from the ground up, again.
Big lesson is to have backups. Lol