I think that google is going to release a new reference device that has a mic built directly into the box so that you can have your tv always on and have it function as a google home with a screen. Considering Google's focus on google assistant and google home and the integration with the casting interface, I think this would be the next reasonable step for the platform.
Shield tv, has this already with the shield controller mic. they still haven't implemented it though, yet. 2 months battery life of stand by batter with he mic always listening.