An available desktop GUI to build an image on a device would be useful in cases for when LuCI Attended Sysupgrade or the The OpenWrt Firmware Selector is unable to be used.
The only thing I would request to have would be the ability to add any user-added packages from their current router.
As to how, previous ideas mentioned were a copy-paste function in LuCI, or a config file to drag and drop. Otherwise, thinking out loud now, perhaps displaying on the GUI a command or script we could copy and run via ssh to get the packages list from.