It would be handy if admins could move discussions between a parent group and it's subgroups. There would be an option in the discussion dropdown called 'Move discussion', which would open a modal. The modal would say "Where would you like to move this discussion to?", with a dropdown listing the name of the parent group and all it's subgroups.
I'm thinking of it as a feature only available to someone who is an admin of the present (sub)group and the destination (sub)group. What do you think?