Add ability to Extend by Formula
Grouping by the result of Formula is very robust - it is possible to produce literals as well as arrays.
If it wolud be possible to Extend by Formula instead of the links and subtasks and Epics only, it would give the power of extending based on the content of the Jira ticket, which may contain i.e. fields with issue list.
It would by possible to construct deep structures with issues not linked to other issues but having them listed in some fields.