I'm planning on running a one-shot in September set during the Horus Heresy, specifcially about the Imperial Fists. I've considered the issue and I overall prefer Dark Heresy's rules and find it easier to make a party that feels individual and era appropriate using it than purely Deathwatch. Therefore I've modded some rules slightly to allow play as an Imperial Fists Astatres. Bear in mind these rules will only be used in the context of everyone playing a Space Marine.

Imperial Fists Rules (Applied instead of Home World)

Characteristics Modifiers

+Toughness, + Intelligence, -Fellowship

Fate Threshold

3 (Emperor's Blessing 6+)

Legion Bonus

Siege Masters : The Armour Points granted by cover to the Imperial Fist's target counts as only half it's value. In addition the Armour Points granted by cover to an Imperial Fist is doubled so long as he remains stationary.

Legion Aptitude

Defense

Wounds

15+1d5

Astartes Background

Starting Skills

Athletics, Awareness, Common Knowledge (War), Dodge, Forbidden Lore (Adeptus Astartes), Linguistics (Low Gothic), Navigation (Surface), Operate (Surface), Parry

Starting Talents

Ambidextrous, Bulging Biceps, Legion Weapon Training, Heightened Senses (Hearing, Sight), Nerves of Steel, Quick Draw, Resistance (Heat, Cold, Poison), Unarmed Warrior

Starting Equipment

Legionnaire Power Armour, Legionnaire Bolter or Legionnaire Bolt Pistol, Legionnaire combat knife, four magazines for starting weapon

Background Bonus

More Than Human : Gain the following Traits: Amphibious, Unnatural Strength (+4), Unnatural Toughness (+4)

Do these seem reasonable?

More input later, so far like the pitch

Considering the background bonus is very powerful by itself, do you intend to give an aptitude option? As it is a generic Astartes rather than specifically an Imperial Fist, maybe Offence or Finesse would be appropriate.

Side note, do you intend to bump up their base characteristics by ten (ie. 2d10 + 30) so that they feel sufficiently space marine-y? I like what you've done and it's interesting to see your conversion to DH2. I've tried converting DW to OW and it sort of worked but this seems far more flavourful and simpler.

The Background there shouldn't be one if everyone is playing a Space Marine. Rather, that collection of abilities should be available to all relevant characters. The idea's otherwise promising, but the intention of a background is to allow a particular axis of choice in addition to Home World/Role.

My recommendation is that before you start designing more stuff, determine what you want each category of component to give. How many skills if any, how many talents if any (and of what tier), how many aptitudes and of what category, how much specialist gear, and so on.

What NFK said.

This is stuff that you should award every character at the end of character creation, like an Elite Advance (Adeptus Astartes: Imperial Fists) - and have them do whatever else they like for chargen.

Make this the "Space Marines Home World"

Create the "Chapter" background

And finally give them a role (Devastator, etc.)