How to use anything to generate thumbnail image of anything in explorer

Yep, that's probably idea of https://github.com/kenjiuno/ThumbGensPack/ made by japanese developer. Thumbnail generation happened in background by command-line application that feed 24bit bmp image, this non-blocking way is safer than using dll but at the cost of spawning repetition (and windows watchdog, antivirus scan and so on) delay of the generator since it's not a "server" process (for example like what RWThumbnails did).

The possibility is endless though you could generate thumbnail for text (e.g imagemagick), svg, image, audio, video, or virtually anything then piping or chaining the processes as long as it produce bmp. More importantly the shellextension works with XP :)

Give it a try!