To-do

CAN is a protocol still widely used in the vehicle industry and some idea of how it works would be relevant to anyone interesting in working in that field. The CAN bus also works over much longer distances then either I2C or SPI And the field is huge...

Next, there are number of home-automation protocols of varying degrees of functionality with which people should have some degree familiarity at least to understand why not to work with them... Two that I have been exposed to are KNX (my house is runs it) and EnOcean which looks very interesting on paper. And, of course, who could forget the perineal favorite - Zigbee.

Then there is the burgeoning technology of Grid Protocols most notably OSGP https://en.wikipedia.org/wiki/Open_smart_grid_protocol which - while complicated - provides some insight into building and designing grid sensor nets which have lots of interesting applications.

And there are a LOT https://en.wikipedia.org/wiki/List_of_automation_protocols of others with very specialized applications.

Bibliography

This article discusses in great detail PCB milling with loads of information on V-engravers.