As I recall Tod Howard went on an interview almost immediately after that trailer and outright said that any real development on the next Elder Scrolls wasn't happening yet and wouldn't be till Starfield was done. Now that Starfield is almost out the door I'm sure more resources will be shifting over towards ES6 soon but that means development is barely beginning. They did claim they put Starfield development on pause to build a feature they wanted to include in ES6 though so they likely have at least some basic concept work done on some level.