For example, if I'd like to sort a list by an "Initiative" custom field but there is an order in which tasks must be done within that initiative that would be helpful to display. I may also have already grouped by status to differentiate between tasks for each initiative that should be done in the near future versus those that are further out.

Users can already rearrange tasks when a field is grouped, but not when a field is sorted. However, only one field can be chosen to group by and the same behavior is useful when tasks are sorted by a field.