Adding .fxg format in Affinity Designer could be quite useful. .fxg or Flash XML Graphics as it is called is an interchange format that was developed by Adobe to let Adobe Creative Suite Applications like Flash,Illustrator,Photoshop,Flash Catalyst,Fireworks,Flash Builder etc could use an interchange format that could be imported and exported without losing details between the Applications,similar to SVG. It is based on Adobe's Flex Framework(An opensource frame work called Apache Flex Framework,now handled by Apache Foundation). .FXG files can be exported by Adobe's Flash Professional and Illustrator Applications. This means that if affinity can provide import and export of this file format,then it could be used as a workflow, Secondly,.fxg format can also be used for theming components in Flex applications,which is a good framework for developing applications. The specification for .fxg can be found on the following URL:
https://sourceforge.net/adobe/flexsdk/wiki/FXG%202.0%20Specification/