Battery life. With the battery and cpu efficiency improvements in the last 10 years, if the features and other specs stay the same then battery life should be incredible. I think month-long battery is likely possible.
Improved voice recognition and AI features. Pebble had voice recognition but it sent everything to a server to process. Now they could run speech-to-text on the watch itself or on the connected phone.
More durable buttons. A known issue with the Pebble 2 is eventually the buttons turn to mush.
What I’m most looking forward to on a new Pebble: