For better or worse, the design is staying exactly the same.
It's been a hot minute (read, 3 years) since Google updated the Chromecast, but according to a new FCC listing, it would appear that a new model could be on its way in the near future.
According to the filing, this new Chromecast will be the first to support Bluetooth. The current model does have a Bluetooth chip, but it's only used during the initial setup process and can't be enabled by users afterward.
The FCC documents show that Google thought about just sending a software update to the exisitng Chromecast to enable this functionality, but "because this new model 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."
Along with Bluetooth functionality, the new Chromecast will also offer improved Wi-Fi reception with 5GHz networks:
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.
Google might announce the new Chromecast at its upcoming Pixel 3 event that's expected to take place in early October, but seeing as how the design for the updated model will be identical to the current one, it's entirely possible it'll simply be sold in place of the existing Chromecast with no fanfare surrounding it.