Matt Birchler has published his list of requests for watchOS 5, and I wholeheartedly agree with all of his major feature ideas. His top request is identical to my own: letting third-party apps populate the Siri Watch face introduced last year.
Essentially, Apple should be making the Siri watch face the smartest, most useful watch face someone can choose. It already is the smartest, but to be useful to everyone, they need to make the apps people are actually using work with it.
I've been using the Siri face nearly non-stop since installing the watchOS 4 beta. Because I use a lot of first-party apps, it still offers me enough value to be the best Watch face for me. Once third-party apps can tap in though, it could end up becoming the best face for everyone.
Pair Siri face improvements with Birchler's other major requests – always-on Watch faces, an Apple Podcasts app, and further updates to Activity and Workout – and watchOS 5 would stack up to address all my outstanding issues with the platform.