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Raven Guard 0.4===========
Pursuant to the recent spate of SM chapter lists and recent conversations on the SG Board about Raven Guard, I thought I'd take a stab at it.  I had tried it before, but in the end simply didn't have enough to distinguish it from the stock SM list with its extensive drop/air assault abilities.
My current thoughts run to:
No "15cm" Garrison.  Scout units only.  Ravenguard are constantly short-handed and approach warfare with their Eldar-ish "Lightning Strike" doctrine.  They would never sit and hold a position if they could help it.  Scout formations would be infiltrators (lower case "i") deployed to guide in the main raiding force rather than "hold this hill" stuff.
Raven Guard Assault.  Essentially Assault Marine stats, without jump packs.  Able to use ground transport for a slightly bulkier ground assault formation and drop pods.
Raven Guard Heavy Scouts.  Something between regular Scouts and Tacticals.  They would not be available on their own, but as an upgrade to the Scout formations to make the formation heavier and reflect the fact that Raven Guard are capable of deploying heavier assets guerilla style.
RavenGuard Drop Pods.  A special rule that Raven Guard Drop Pod formations may reroll scatter dice.  This would give them a better chance of hitting where they plotted or keeping a favorable scatter.  It would even let them reroll a "hit" in an attempt to gain a favorable scatter if they needed it.
Other ideas:
0-1 restriction on Armor Formations.  I'm a bit torn on this.  In 40K they have the "Flesh over Steel" restriction which inhibits their use of vehicles, but I'm not so sure it really fits the background.  It would also limit the presence of Hunters.
Increased Imperial Navy support.  Would reflect additional reliance on orbital assets.  I don't know of any background that addresses this either way.  It just seems like it would make sense.  It would also allow better air support if Hunters end up lacking due to the armor formation restrictions.
Increased spacecraft abilities.  Pretty much a whole cloth idea.  Basically, just to reflect the same idea as above - increased reliance on orbital assets and to encourage drop armies.
Planetfall Speeders.  Pretty much speaks for itself, I think.