I agree that the special API that ThumbCreator uses to provide config widgets is a bit weird. I don't really know anything about KCM (does that add a GUI in ksystemsettings?), but I think it's important to be able to configure the thumbnailer in some way. Even if it's just raw config files without a GUI. The average user doesn't currently need the config options that are present, but I can imagine advanced use cases that do. For example: Over here someone suggested pulling thumbnails from the embedded metadata or even from static image streams of a video file, instead of generating them from the video stream. That would probably a on-by-default option, but I imagine there are cases where users want to disable that, e.g. if the embedded thumbnails are low-quality or very generic. That's one of the options provided by the upstream ffmpegthumbnailer.