A Raspberry Pi powered by something similar to the M1 or A14 bionic processor would be something. The Broadcom 1.5 GHz 64-bit quad-core ARM Cortex-A72 that powers the latest raspberry pi, is nothing compared to the new processor beasts from Apple.
We already have the Mac Mini, but an A14-based SBC would be less of a stretch than an M1. A picoMac, so to speak? That would almost inevitably run MacOS by default rather than Linux, though. The GPU is make or break for Linux. No GPU means it's just another server in a different form factor.
Let that thing have more standard IO, and run Linux, and it'd be a heck of a fanless machine.