Motorola did a sort of add-on thing with their Moto Z some years back. It had swappable back-plates. They launched an extended battery, a speaker, a camera with larger optics and mechanical zoom, a projector attachment... some sort of amazon-branded smart-speaker attachment... they were supposed to kick off an add-on ecosystem but I guess it turned out most people weren't really bothered.

Not quite what you're asking for, it wasn't standard bus connectors or anything.