If you want to remain within Google's ecosystem when smartening up your TV, you have a couple of options: either buy a Chromecast to control the flow of content from another device, or buy a box running Android TV made by a third party. What you can't do is merge the two and get a Google-made dongle device running something heftier than what the Chromecast has to offer. Such a device exists, but Google won't be selling it to you.
With Google I/O currently going on the company has decided that it will start selling a product running Android TV, which it hasn't done since it discontinued the Nexus Player. The problem is that this product is the ADT-2 which is only going to be made available for developers. While it looks very similar to the current Chromecast design, comes with an Assistant-capable remote, and runs a developer preview of Android P, Google says it has no plans to release the device to consumers.
The dongle itself has Amlogic a S905X processor, 2GB of RAM, 8GB of storage, and support for 60FPS 4K video, and HDR, which puts it capabilities roughly on par with the newest Amazon Fire TV dongle and Roku's Streaming Stick+.
Google only has a limited number of the dongles available, with priority going to developers who actually have Google Play accounts. And before you try and pretend to be a dev to nab one for yourself, Google is asking everyone that applies to fill out a questionnaire about their app development work. So unless you have bullshitting skills that can outsmart a multinational corporation, you're out of luck. If you want a fully-fledged TV experience in a discrete and compact device, Amazon and Roku look like your best options. [Google via Lilliputing]