Both Nike Run Club and Slopes offer the ability to start a workout on your Apple Watch and for the data the be viewable live on your iPhone (and vice versa). This would be fantastic for those of us who want a larger screen to look at stats during a workout. To on even further, Live Activities would be a great addition. Here is how Slopes implements it with their live workout data from an Apple Watch.
Example: You start an outdoor cycling workout on your Apple Watch but want to use your iPhone to navigate on your handlebars. Would be great to have the data live and viewable on both screens in order to see speed, distance, etc.
As of now, I have to start 2 simultaneous workouts, one on my watch for capturing HR and one on my phone for live speed on my handlebars, and then delete one after. No bueno.