Is there really a need? Most serial protocols can really be considered 'open'.
For low bandwidth there is RS232/UART/JTAG. Then there is I²C/SPI for networked. I²C ranges from 100kbit/s to 2.3Mbit/s. And SPI goes to 10Mbps/30Mbps.
For higher bandwidth stuff there is whatever you want over USB, Bluetooth, 802.11BLAH Wifi. Etc... Afaik USB can be considered 'open', you might need to 'register' for a certification/vendor id/logo but from what I understand if you don't want those you don't need to bother. There are also http://pid.codes/ and other organisations that are giving away free pids.
For low bandwidth there is RS232/UART/JTAG. Then there is I²C/SPI for networked. I²C ranges from 100kbit/s to 2.3Mbit/s. And SPI goes to 10Mbps/30Mbps.
For higher bandwidth stuff there is whatever you want over USB, Bluetooth, 802.11BLAH Wifi. Etc... Afaik USB can be considered 'open', you might need to 'register' for a certification/vendor id/logo but from what I understand if you don't want those you don't need to bother. There are also http://pid.codes/ and other organisations that are giving away free pids.
There is the Wishbone bus. https://en.wikipedia.org/wiki/Wishbone_(computer_bus)
But that's got quite a few pins. It allows for on chip networking as well as external stuff. Also RapidIO (which has heaps of pins).
There is a RISC-V debugging standard, but I think that's protocol agnostic.