I don't know if this counts, but I'd love to try writing a fairly hard sci-fi take on the Warhammer 40K universe. I've read the time line, and there is a bloody long gap between today and the fanciful stuff happening - E.G. unlike Star Trek, which has FTL travel being discovered in about 40 years, 40K doesn't introduce it until the year 15000. For all that intervening time humanity has a more-or-less unrecorded history in that universe, and there's no reason why you couldn't write a story that was fairly realistic in it. Grim-dark plus HSF might be a bit much, but I'd be interested to see it (or have a go at writing it myself).