The one thing i love the most in WH40k is the lore... but im no purist. I'd prefer, thou, if they kept it as close as they can to the original lore.
What's wrong with the Blood Ravens? I liked them a lot, being different and all. Plus Games-Workshop recognises them as an official chapter, though I can't remember if they were a minor chapter given to Relic or thought up by Relic entirely. EDIT: Whoops, quoted the wrong guy, not used to bottom-replying.
From what has been said by the devs, they are going to kept it as close as they can to the original lore. That makes me a happily camper, also GW has said that they may make the events in game cannon; like how they did with Blood Ravens.
A little licence is okay, but keeping mainly to the lore would be awesome, especially if you have NPC's fitting their respective codex entries.
To elaborate: Try and keep the weapons true to themselves. Having just replayed through Space Marine, yeah the melta-gun was cool, but why oh why was it essentially turned into a shotgun, when scouts already have perfectly good Astartes pattern Shotguns
So long as they don't reach the levels of C.S. Goto (MultilaserMultilaserMultilaser) I reckon we're fine.