Values in drop-downs for filters in reports would be based on the actual results, not all valid values

What is the challenge?

We have a sales territory hierarchy and so the valid values of one field depends on the previous other values chosen. However right now, if choose the USA as an Area, I still have choices like England and Spain for the Region fields, even though no actual values would be returned.

How would you solve it?

Reporting tools like Google Looker look at the actual values from the results returned. It of course takes additional processing but the usability would be much better.

What impact would it make?

If I am a sales manager for USA, I could easily see the Regions in my Area, like Northeast, Southeast, etc. instead of having to look at all 50 regions around the world to filter on