I've been thinking about how to run Horus Heresy using the Warhammer 40k systems. Deathwatch is designed for Space Marines but is a little outdated now and lacks some flexibility when it comes to character building. Therefore I'd draw upon Black Crusade and Dark Heresy 2e to do it.
1) Character generation would roughly follow the Dark Heresy 2e template. The main thing used from Deathwatch would be Demeanours. Characters would have 100 points to spend on Characteristics which start at 30 base (or 25 if a - characteristic or 35 if a +)
2) First element would be replacing the Homeworld choice with a L egion. This would essentially be the same - 2 stats to boost an one to lower, starting Fate and Wounds, and a starting aptitude. I'd even consider every Legion having a minor special rule as well, eg. World Eaters gain the Brutal Charge (2) trait. This would be the hardest section to do as it would essentially require starting from scratch.
3) Standard background would be using the Space Marine starting template from Black Crusade. These rules are a bit more streamlined and match the most recent standard of the rules. Only major change to this stage would be removing the Forbidden Lore (Horus Heresy and the Long War) skill from starting skills. I would probably also grant players a choice of Weapon Skill or Ballistic Skill aptitude at this stage.
4) Final stage would essentially be using the unmodified Role choice. This feels broad enough and gives players freedom of choice.
5) Equipment choices would be taken from the Black Crusade or Deathwatch errata lists. Influence would be used.
6)Power Armour however could be selected from the list in Deathwatch: Rites of Battle, any armour between MkII and MkVI. No armour history as they would only be 200 years old at maximum and are less likely to be passed on.
7) Regular Dark Heresy character generation can be used for anyone else, eg, Rememberancers, Adepts, Astropaths and Iterators.
8) Everyone gets 1000xp to spend.
9) All combat rules, skill lists and Talents use Dark Heresy 2e. This includes the use of Aptitudes. Hordes from Black Crusade and Formations from Only War are pretty much essential.
I would appreciate some feedback. I'd also consider trying my hand at making some Legion Templates but I'm wondering what special rules would be appropriate and balanced.