Currently, next-gen boards can't show sub-task issue types in the same manner it can other issue types, like bug, tasks, etc. It makes it very difficult to understand the body of work that might exist within the board and the statuses of each sub-task. While next-gen now has the "Group By" function which can show sub-tasks, it only can show those sub-tasks when grouping the entire board in that manner. That is not desirable in all situations because you then can't group by other things like assignee or Epic.
It would be great if next-gen boards would allow you to view sub-tasks like it can any other issue type.
It would help to give end users better visibility into the all the issue types, not just standard issue types.