Hax here. As rumour would have it, 6th Edition of WH40k looks to be a game changer. I have a few items that I really really really hope make it in to 6th Edition.
1) The Boxed Set: I really hope that this is Dark Angels (DA) vs. (very chaosy) Fallen Angels (FA). This will bring the cannibals from Caliban back to the fore of the collective imagination where they belong.
What do I want to see in the DA codex? Why thanks for asking. For starters: PLASMA! 1) Deathwing Termies with a Special Weapon Choice of Plasma Cannons 2) A special rule that allows for re-rolls on "Gets Hot!" (perhaps character driven) 3) Something Ravenwing Plasma... I'd also like to see Azrael or Asmodai make DA Vets troops. Imagine an all hooded/robed army on the board. Belial should be at least on the level with the likes of the SM Captain... But I actually picture him something Tycho level...
What should be in the Fallen codex? Well, trick question, actually. IMO, it should be the new Chaos Legions codex, with a Cypher/Astelan character option to make a FA army. You see, FA really turns the 40k universe on its head. It's the best way to play the bad guys who think that they're the good guys; and just might be... maybe. I have to admit that the Descent of Angels book is so far my least favorite Horus Heresy novel. However, it really did make me think about the DA. If you read carefully, you can interpret the massive secrecy and craziness with the orders, combined with the massive time-fudgery in Fallen Angels to mean that the Order was infiltrated by Fallen who were hurtled back in time. That is to say that the order was infiltrated from at least the start of the first novel. Ouch - it hurts my brain behind the eyeballs to think on it too hard. To summarize. The DA were overly screwed before they were screwed over. Ye gads, what might that look like?
Updated Cypher rules for starters... make him a Special Libby along the lines of Mephiston (but a pistol specialist vs. crazy sword expert). There are hints of him being Zahariel from the current HH series. In the novel he has some pretty well defined Terrorsight psyker abilities... Matter Phasing, Crazy Endurance, and Time Slowing. Easy peasy in game terms. Psychic powers: 1) All attacks ignore armor saves and count as rending. 2) FNP. 3) Strike first in combat (Init 10) or move in assault phase. Twin pistols rule from before... Lion Sword that he never uses. Perfect. So, bring him in line with that.
Oh, Astelan... you are a piece of work, no? Astelan should be something along the lines of a chaosy version of Shrike/Sicarius - SM captain level at least (he was actually a "Chapter Master" pre-heresy). Maybe somewhat stealthy and morale boosting. Perhaps allow Astelan to take allies from Codex: Anything-At-All. This would represent his (multiple) successful planetary uprisings with strange bedfellows for allies, and bring some form of allies back to the game (missing since DH and WH updates). Something along the lines of: Must fill required FOC with C: Chaos Legions, optional FOC choices may be taken from any other valid codex. You want Knights of Lupus, add a dash of SW. You want grimdark cultists, add DE. You want Xenos love heresy, add Eldar. You want a leader for the Blood Axes, add Astelan + Ork Troops. You want Genestealer invansion, add Tyranids. You want traitor guard, add IG. You want SCIENCE!, add Tau. I really like this flexibility.
2) Overwatch... See the right sidebar for how we would do it here at LR. It's a tactical element truly missing from the game. Although, I think that I would choose to exclude vehicles from being able to go in to Overwatch.
3) Some sort of small points games larger than KT, but < 1000 pts. A very simple 500 points game like Combat Patrol, but with fewer restrictions and more elegant rules. Let people scale their way in to the hobby in a defined way.
4) Measure if you have LOS. I think that it's dumb we cannot measure distances when targets are clearly visible from the model POV. Rangefinders are pretty simple tech. Even orks could have a "Ragefinda". Nids could call it proximity sensing magnetics.
5) Scenarion that switch up the turn phases.... instead of MOVE1 SHOOT1 ASSAULT1 MOVE2 SHOOT2 ASSAULT2, it might be interesting to try MOVE1 MOVE2 SHOOT1 SHOOT2 ASSAULT1 ASSAULT2. Of course, somebody else needs to playtest this. Not I.

Some say this is Merir Astelan.