After four years with no new versions, there probably won't ever be one.
Even the original 2015 Shield is still one of the top 5 Android TV devices ever made, with the 2017 and 2019 models also taking spots.
I did add the newer remote to my 2015 model as that's a big improvement (other than the annoyingly easy to accidentally press Netflix button, but I remapped that to Show Recent Apps instead).