Request to have a confirmation dialog when deleting a subflow.
Subflows can contain a lot of work and deleting them can sometimes make your heartbeat go up. I know that ctrl-z - undo exists, but the realisation could come later - too late.
The subflow won't actually be deleted until you deploy though will it? So you do already have a 2-stage delete. Then also, remember that the flow file is backed up on each deploy so if you make an error - STOP, pause - go take a copy of the current and backup flow file and then try swapping them over.