It's a real shame that optical or digital never progressed and couldn't handle the full audio and become a plausible option especially since TVs still offer Optical Out and so many audio electronics support digital connections (Toslink/optical and Digital) but not HDMI. It limits connectivity and upgrade options massively.
What should be revised or further developed at the digital interfaces or connections? And why?
Optical has always been just a cheaper version of SPDIF, especially since cable was a lot cheaper. With SPDIF and AES you have data rates that leave nothing to be desired and USB 2.0 is also more than sufficient with a maximum of 1536KHz/32bit with PCM or DSD1024. There is also i2s (over LVDS), which transmits the data as the converter ICs need it, nothing more is possible.
If you play on HDMI, everything is perfect in terms of transmission.
For the rest, especially as far as the possibilities and limitations of HDMI are concerned, you have to thank the people who caused such great financial damage in the entertainment industry in the 70s to 90s that a large consortium decided to take the to prevent the processing of the data in the future if there is no license for this.
That was the birth of HDMI, including the copy protection concept (DRM) for audio and video.
It is absolutely forbidden to make the digital data accessible except in a limited way via optical or SPDIF.
If you are not part of this consortium, you will not get the necessary documentation or ICs to develop or produce a device with HDMI.
But since there are now many devices that can process HDMI, what exactly is still missing?