I have a Customer (custom) field and need to analyze on a regular basis the number of hours used for each customer. This can of course be reached with multiple timesheets, but as the customer base increases, this becomes impractical.
I suggest that the user has the option to choose the field(s) used for grouping on his custom timesheets. In that case, when defining custom timesheet, the user picks an issue field he wants the grouping to be performed on.
In fact, current implementation is really close to handling this via the Issue Fields selector, where you can select multiple fields. What is missing is "only" the capability to group or subgroup on the chosen list of fields. For my case, I would choose the customer field and get exactly what I am missing.