The required open protocol already exists along with support in Netflix and some other apps, DIAL. What is needed is a convenient packaging of a DIAL server along with an open platform for developing applications for use on the TV.
As I understand it, even the Netflix TV client has custom extensions to perform more than the most basic functionality.
You also need to get that app approved onto the TV, which, chances are, require an update. Sadly that means that you're likely never getting support for the majority of the devices already in the field.
DIAL's time has come and gone. Adoption during the SmartTV hey day 5+ years ago may have made it a contender.
There is no need to get anything approved onto a TV. The apps would run on a media device, such as a Raspberry Pi with the appropriate software, connected via HDMI to the TV just as the Chromecast does.