Many years ago I ran a couple of TTRPG mashups of Warhammer 40k and Dune. The emperor was much the same as 40k, the Sardaukar were marines, the Bene Gesserit were a plot manifested by Tzeentch, the Butlerian Jihad explained the lack of computers, etc., etc. It was - if I do say so myself - pretty awesome.
Now my kids are getting into role-playing and not interested in fantasy so I'm thinking about reviving my old mashup for them.
In the old days I used the 1st ed. Rogue Trader rules which seemed like the beginnings of an awesome TTRPG before Games Workshop committed fully to selling lead, then plastic, figures for skirmish wargames.
Is there a newer version of Warhammer 40k that is still TTRPG focussed, or should I dust off my old, loose-leaf copy of 1st ed. and work from that?
I know it's not directly what you asked for, but Stars Without Number is a fantastic Sci-Fi RPG, and is super open like Rogue Trader was. Honestly, reading it reminded me of when I first read Rogue Trader as a kid.
The single worst thing about it is that the cover of the book doesn't look very exciting when compared to other RPGs.
FWIW, you could easily map the lore of 40k (either the fun RT lore, or the modern stuff) onto the SWN game system without breaking anything.
Why would you want to use a different system than the original RT? Let's just say that in the 30-odd years since RT came out, much better game mechanics have been developed.