I'm interestet on the fitness App on the Watch. How exactly is the Calorie calculator?

Hi Tom

Apple Watch uses various sensors and data sources when estimating calories and other results. In some cases, it will rely on your arm motion to estimate your pace and, in turn, calories burned. In other cases, it may use data from the heart rate sensor and/or from your iPhone.

For example: if you record a workout using the built-in Workout app and choose Outdoor Walk or Outdoor Run, the watch can use sources including the accelerometer, heart rate sensor and - if you also take your iPhone with you - Location Services (GPS) data (to help estimate your distance and pace).

Estimates of calories burned also take your personal information into account (age, gender, height and weight).

Calibrating your watch can improve the accuracy of calorie and other estimations made by the Activity app and Workout app.

More information:

http://www.apple.com/watch/health-and-fitness/

Use the Activity app on your Apple Watch - Apple Support

Use the Workout app on your Apple Watch - Apple Support

Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support

You are most welcome! 🙂

I'm afraid that no-one here can tell you anything about Apple's unannounced plans (this is a user-based technical support community) and it is also not allowed here to speculate.