There are many different alternate rules I have been testing for fixing the "rank" system. One of the best ones so far is the "rank jumping" system, whcih is an elite advance that allows you to jump from one rank to the other, despite not having the XP requirement for it. It costs 100XP to rank jump and the rank you jump out of is now considered a missed rank (as per alternate career ranks) and therefore the advances there are still available, but a a cost of an additional 50XP each. Rank jumping explains, to me, the idea of those we all know who have higher positions (in the military, at an office, in clubs, groups or whatever) then others, despite knowing less and having less true experience. It is the 20 year old noble who because of blood ties and connections is made a captain of a company of IG troops. It allows that individual to get access to the skills he needs at his new rank, but at the cost of not being able to learn the basics of his job as easily.
An additional part of the Rank Jump is the Rank Halt, and this is the guy who never seems to get an advance at his job, the continuous grunt, the overlooked scholar. These guys are truly experts in what they do but never move on to the next levle of their career or path. Whenever a character reaches enough spent XP to go to the next rank, he can opt to not advance, go back to the beginning XP for that rank (but has all of his past advances still count) and gains the Talented (skill of choice) talent as payment.
Anothing thing I am working on is drawbacks, which enables a PC to take certain "experience based" penalties (enemies, fears, Insanities, debts and so forth) that nets them some XP that can be spent to begin with.
I also allow players to spend 100XP to make their "treat as basic skill" skills from origins and backgrounds as trainied skills and allow the Rogue Trader "doubled" skill rule.
Where the original career paths in Dark Heresy Core diverge (example, Rank 6 Guardmen) I am thinking of treating the branches as alternate career ranks of some sort. I am considering lowering Scout, Shock trooler, Marksman, Sniper and Storm Trooper to ranks 4, 5 or 6 instead of 6, 7 and 8. And doing away with Veteran as a "Base" career rank for Guardsmen and turning it into a Rank 2 or 3 Alternate Career rank. On that same note, Assault Veteran would be a rank 4 or 5 continuiation of Veteran.