Will Garmin add a feature like Whoop or Oura ring?

Good question. Afaik optical heart rate sensors can measure HRV reliably only when staying relatively still. Thats why Oura tracks HR/HRV only when sleeping (and also battery concerns). Garmin says that Stress score is calculated from HRV so low Stress scores correlates to high HRV.

So, what I think what's going on is that Garmin thinks their optical HRV data is not good enough to show the exact value BUT it can be compared to historic data per person. And that's why it's somewhat obfuscated for example under Stress score.

Optical heart rate sensors estimate HRV from pulse wave variation (PRV), which is naturally less well-defined than HRV, The electrical signal from the heart provides clearly defined spikes that the straps use, whereas the pulse wave is more sinusoidal with with no clear spike, rather a smooth curve. From that smooth curve the sensor has to detect the colour change necessary to estimate the variation between peaks.

This would be awesome.  I am a watch guy and own a handful of older mechanical watches but also value my data.  I find it hard to take off the Garmin even though I want to wear my other watches.  If they had a ring option for basic data during the day like HR and steps I'd buy it in a heartbeat for the data while I wear my nice watches. 

If not a ring a simple band. No screen needed just HR sensor for tracking sleep, steps etc. 

I do agree. I take off the Fenix while sleeping, find it to big. A simple band as a complement to track sleep would be perfect!