The second-generation Chromecast, which is still the current model, was released in 2015. There hasn't been much of a need for hardware refreshes, especially since the original model still works just fine. It was already known that Google was working on a refreshed model, but now additional details from FCC documents are available.
Back in May, the FCC first published documentation for a new Chromecast, with an ID of A4RNC2-6A5B. For context, the second-gen Chromecast has an ID of A4RNC2-6A5. The filing included this note:
This application is for an enhancement to the existing product approved under FCC ID A4RNC2-6A5. A software change is being implemented to enable legacy Bluetooth operations. Because this new mode requires a new equipment code (DSS) not covered by the original certification for FCC ID A4RNC2-6A5, the new version of device requires a new FCC ID. For clarity, devices bearing FCC ID A4RNC2-6A5 will not receive the software update to enable legacy Bluetooth operations.
Simply put, Google wanted to enable full Bluetooth functionality for the existing model, which requires applying for a new FCC ID. However, an updated document shows that there will be at least one hardware change:
This device has been changed to include a 0.5 mm trim on the 5GHz PCB antenna trace that increases the 5GHz maximum antenna gain from 2.1 dBi to 4 dBi.
While decibel-measurement is a bit difficult to calculate, with different scales for power vs voltage, this appears to be a roughly 55% increase in maximum possible gain when using 5GHz Wi-Fi. Best Buy still displays the old FCC ID on its Chromecast listing, so it appears this updated model isn't being sold yet.
Last month, a report from Kotaku revealed that Google is working on hardware for its upcoming 'Yeti' game streaming service. It seems likely that these hardware improvements could turn the Chromecast into an inexpensive client for Yeti, much like a Steam Link. 5GHz Wi-Fi has a lower latency than 2.4GHz, and full Bluetooth support could allow a game controller to be connected.
Even though Google does have its annual October 4 hardware event coming up soon, it's possible the new model will quietly replace the 2015 Chromecast, since the design is identical.