I've done it. I've designed the 15mm miniatures skirmish game for Those Dark Places. You can use RPG characters at a push but the basics are the same.
I didn't realise I'd already designed so much of it. I just need to playtest it to hell and then write it up properly.
It won't be released any time soon - there's other stuff for me to finish - but that's another project that's on its way!