Personally I would like to see the following:
A break down of the astartes implants and an actual game function asigned to each one. A bit like the techpriest trait. More than simply 'unnatural strength and toughness'. Back when I playtested DH I created a list of the different organs and assigned them an ability on the assumption that a character started from a scout and moved up.
This is I feel an important aspect the RPG should address. Those implants are more than just a bonus to a stat, they are the reason a marine exists. They hold a holy reverance for them.
Reflection of indoctrination beyond just 'Mental Fortress' or 'Iron Discipline' talents. A marine's psychology is very specialised and also specific to their chapter. These I feel can be reflected in the rules of each character.
The other is individualisation of power armour. I would like to see power armour broken down into location with different marks offering different modifiers and upgrades. Ie a marine might be wearing Mk IV greaves, Mk VI pauldrons and Mk VII torso, arms and helmet.
Power armour is like a spiritual partner for a space marine, their other half. If a marine could be married to anything it would be their armour. They have an intimate knowledge of one another that produces a warrior greater than the sum of its parts. That reverence should be reflected in more detailed armour rules I feel.
Afterall, marines have a much narrow range of options than a normal human warrior so extra focus on specific important wargear items could go a long way to fill the deficit.
I can't imagine they won't have chapter traits as part of starting creation. It's an obvious choice. But the above are my biggest wants because to me those represent a space marine more completely than anything else. Not the gun, or the chainsword, but the implants armour and indoctrination.
So, what would you like to see in the Deathwatch book?