How can I configure HIIT to plan a training session with haptics telling me when to start, stop, etc like Garmin watch does?

Hi

That is not currently possible via the built-in Workout app.

The HIIT activity type is available within the Workout app under watchOS 4.x:

• About workout types - Apple Support
• Work out with your Apple Watch - Apple Support

Whilst a HIIT activity type is included, it is not possible to set a schedule of intervals using the Workout app.

For that purpose, you may find that a third-party app meets your needs. Apps that you may wish to consider include, for example:

Intervals

• "Apple Watch - The most advanced Interval timer for the Apple Watch"
• "Highly Customizable"
• "Standalone Apple Watch workouts. Leave your iPhone at home!"
• Intervals - Workout Interval Timer on the App Store

Runmeter

• "Built from the ground up for iPhone, iPad, Apple Watch and iCloud, it makes your iPhone a powerful fitness computer — with maps, graphs, splits, intervals, laps, announcements, zones, training plans and more."

• Runmeter Running & Cycling GPS on the App Store

To browse and search for these and/or other third-party apps with the results pre-filtered to include only those iPhone apps that also include an app for Apple Watch, use the Watch app on your iPhone. Apps can be searched by keyword or app name via the Search tab. Follow the instructions here to add and use compatible apps on your Apple Watch:

Use apps on your Apple Watch - Apple Support

If you would like to submit a feature request to Apple, then you can do so here:

Feedback - Apple Watch - Apple

You're welcome! 🙂

I'm afraid that no-one here can tell you anything about Apple's unannounced plans (and it is not allowed to speculate here).

I suggest finding and using an app now that works best for you.

Glad to have helped! 🙂