@myst I just finished modifying everything for my needs. Here things I did, maybe it helps for optimizing future releases:
- compressed 1080p backgrounds for performance optimization on Pi3
(because quality still looks better than 720p version and images are smaller) - compressed audio files of used systems
(cannot hear a difference and maybe better for performance on Pi3) - replaced audio file of system psp by a louder version
(still don't know how to normalize a bunch of ogg-files to same volume level, I had to do it with mp3 and converted it afterwards) - prboom background + sound used for system pc
(copied to folder to activate art for Dos) - neogeo background + sound used for system fba_libretro
(copied to folder to activate art for Final Burn Alpha) - custom.xml of most systems (France -> Europe, etc.)
(I modified text of MAME and fba_libretro system a little too) - replaced content in data-folder with files previous version 0.3
(can't tell, but I liked the clean gamelist view of version 0.3 much more, so I reactivated this one + compressed backgrounds like there too)
If you need more information or any for example files, just contact me.