I would consider it perfect if it could do the following in order of preference:
File browser to allow easy selection and loading of disks, tapes and virtual carts. If I can only have one it would be to be able to load tapes (.tap) so we can enjoy the loading screens and music
Support for the db9 to usb retrofun! Adaptor and 8bit Atari based joysticks like the zip stick
Be able to be set to auto boot into a particular game or the file browser
I think thec64 is fantastic and is the first implementation to get the smooth scrolling with no tearing and almost perfect audio representation. Truly great value and I am really looking forward to where you take its development.