More Of: Stories that offer us 'more of ' the 40K universe: focusing on war/grimdark (bolter-porn of Ultramarines V Orks), extending exisiting narratives (see below) or the stories of existing characters. I would also include within this attempts to write in a self-conscious BL style (maybe for publications, maybe for security).
More From: attempts to situate a story within the W40K universe, but to tell a story different than the typical BL-fare (what I termed 'Refocusing the Camera' below)
More Than: (thanks for Sholto for suggesting this) where the author rejects the canon and attempts to tell a different story (the whole Horus Heresy was just a dream and Horus wakes up from a nightmare a la A Thief in the Night) or where they try really skew the canon in a different direction. Im guessing that humour could also be fitted in here as the author seems to be saying 'forget all this grimdark, lets have a laugh at it all'. I would also include stories here that tried to re-read non-40K stories through the lens of 40K. Possible examples:
Primarchs by Lastie (a comedic take on the Primarchs)
It's All in the (Chaotic) Family by Vyce Dryke (another 40K comedy)
This is all pretty fluid though still...