Here is an article on how to add add custom icons to any device type . . . if there is demand for it i will make several door icons and upload them here ot be saved and used with the custom icon hack . . . just let me know 
However yes i agree there should be a generic DOOR ICON as standard, for instance on doors you have open close sensors on etc . . . maybe we could get together a list of icons we would all like to have added, and the top 50 say get made and added in . . .
kyle