Essentially? A "Reverse Ascension" - basically the equivalents of ranks 1-4 in DH for Rogue Trader PCs, if only as a means to help build from the ground up and also because it might help compatibility. Bonus points if we get the 'ascension' to at least Rank 16 equivalents on the other end. But really, I'd like to see how someone progresses from whatever to Arch Militant or what a Rogue Trader learns and does before they assume command. That sort of thing. Or how a DH group could then transition into Rogue Trader if a GM wanted, instead of Ascension.
By the same Token I'd probably say the same thing for Deathwatch wouldn't hurt either.