[@LordZell]

My present idea offering is this: 

You run a company with space marines but do whatever it is you wanted to do with your IG, with them instead. (SM scouts maybe?)

OR you can, for instance, run the chapter serfs. Chapter serfs are logistical support in this game, however i promise to make them interesting. For one thing the department that holds the reliquary armoury is under logistics, you have a space marine honour guard to protect it. You also control the naval armsmen and naval officers who pilot the ships who are all human. Tech-Priests, you share control of with Tech-Marine and Apothecary, as per treaty of Olympus, Tech-Priests are kind of independent and exist in their own little bubble. That said, they are indentured to the chapter, they wear our colours and everything. Chapter serfs are also, sometimes failed space marine aspirants, not really sure if i'll allow this though... If the Primarchs geneseed doesnt take that's usually a bad omen.

Those are the options, i dont want to make the game about a regiment. Regular humans against impossible odds is cool and all, but you havent even asked me about the odds facing your marines... are they impossible maybe? Is it that one? Yes, its that one.