However the process of robo-insemination is far too complex for the human mind!
A knee high fence, my one weakness
I know we do not want to make mention of "Ultramarines (2010)" but some of the casting decisions I could not agree with more.
I would easily take John Hurt to play a psyker and Sean Pertwee auditioned just fine for a Captain.
To get a nice contrast you could have Terrance Stamp to play a "good" Commissar, Bill Nighy play a 'bad' one. I believe Abnett said he based the appearance of Gaunt on Goran Visnjic, and while I love his acting he'd really need to clean up the accent. =)
As many have said previously, I could not imagine a 40k film without Christopher Lee whom would be great as a Chaplin, Lord General, Apothecary, Navigator, High Confessor, Archon, Canoness or Norn Queen (his choice of course).
Oh did I need to cast a space marine specifically? Hmmm. I pretty much love every roll Idris Elba takes on, so he'd be my first, with Daniel Craig as a secondary.