There would have to be a lot of rewriting to make playing Space Marines a balanced choice versus a hive scummer or Imperial Guardsman.
And who spoke of balancing a space Marine against a Hive scummer?
It's a common topic of complaint with Black Crusade - at character creation, you can choose to be a super-human in power armour, or... just a guy. The early WH40KRP games tried to get around this in a way that allows for cross-over gaming by starting each line at a different XP total, with the idea that the same total would represent a comparable power-level across the different lines ("A Rank 7 Acolyte, a Rank 5 Rogue Trader, and a Rank 2 Space Marine team up for a special mission..."), but I don't think anyone feels that it worked out as well as initially hoped...
I'd love to have a Unified Rule Set, with different options for overall starting power level (Low, Medium, High) and character options scaled correctly within each level (so, Low levels would be careers like Scummers and Guardsmen, while High would be things like Inquisitors, Temple Assassins, and Space Marines), but I agree with Lynata: that just won't happen.
Well, maybe if Disney decides to go into the RPG business and yanks the Star Wars license away from FFG. Then that would be a great '3rd Edition' gimmick to re-energize the 40K line!