Hello,
Something I've always struggled with in Forms Reports is trying to set up reports that have multiple levels of grouping; more often that not it is a tradeoff between one thing or another to get as close as possible.
For example, I have a process that is divided up by location, and with the current options I can group by Date or Location, but not both.
I've mentioned before that date filtering would be a fantastic and sorely-needed addition for effective reports so I could allow our managers and supervisors to look at data for a specific date or date range without having to generate a bunch of snapshots.
In addition to that, adding the option for multi-level drilldowns would be a great supplement/alternative to that kind of feature.
For example, I could have the initial report group all activity by Date so the user could select the date they want to review (while on the subject, the ability to set a default sort column/order would be amazing too).
Next, instead of getting a full instance breakdown, they could be taken to a second summarized table grouped by location before getting to the final drilldown with all of the filtered instances.
It's possible that a more accurate description for this would be "Group By, Then By" but from a user perspective it would look essentially the same as a drilldown except there would be more "levels" before the see the full list of instance data.