I half-expect that I'm going to be seen as some twisted heretic for this, but I figured I would throw it out there and get an idea of who thinks what. I'll preface this by saying that I have a decent idea of the 40K universe and all (I'm coupling the various game books with the army codices and the like), so I'm not just trying to ignorantly blunder off into uncharted territory.
My roommates are potentially interested in seeing what the 40K universe is about. One of them is female, and I'm not very keen on trying to mix Sisters of Battle (via Ascension) with Space Marines, because I still have the feeling that the Astartes are gonna run the show, and I want to be certain that I'm not forcing her to play a male character and therefore be outside her comfort zone. So, I came up with the following, courtesy in no small part of the Rites of Battle:
738.M41 -- As part of the recent Twenty-Sixth Founding, the High Lords of Terra found themselves with a conundrum: the Emperor's Tarot threw them a curve and required the formation of a unique Space Marine Chapter, composed solely of females -- what sort of destiny this Chapter might have under the Emperor's protection is anyone's guess. This sends the High Lords into a flurry of activity, until finally an agreement is worked out with both the Inquisition and the Adepta Sororitas to monitor the formation of the new chapter, named the Golden Spears. As part of their monitoring and agreement, the Adeptes Astartes at large are not told of the all-female nature of the Night Spears, at least not initially.
The Progenitor gene-seed, perhaps surprisingly, does not come from the Ultramarines, but instead from the White Scars. Their assigned homeworld is a feral world uniquely suited to the needs of the Night Spears -- the human population is extremely female-dominant, and yet manages to thrive in spite of the gender inequality. As the Chapter formation proceeds, the Night Spears see very limited action, but fortune is with them: the inaugural Chapter Master (the titles remain the same, in keeping with the Codex) manages to win a rather stunning victory over the forces of Chaos, specifically the Alpha Legion; this is made all the more amazing in that a Daemon Prince of Tzeentch was involved and also defeated.
As of 817.M41 (Deathwatch setting, IIRC), the Golden Spears are not quite at nominal strength, but growing close. The Chapter takes many cues from the White Scars in regards to combat doctrine and general demeanour, but charts its own course -- members are trained extensively with the Astartes Power Spear for use in combat, and it has become a traditional sight for the Golden Spears to field spear-wielders. On occasion, the Golden Spears have assisted the Sisters of Battle and the Inquisition, and more rarely team with Grey Knights; like the Silver Skulls and the Exorcists, the Golden Spears are aware of the existence of the Grey Knights, due to their ties with the Inquisition, but are still subject to post-mission mind-wipes when the situation requires it.
I based this predominantly off the Amazons of myth, though I did add the feral world the Golden Spears chapter is based upon from Eddings' Belgariad and Mallorean cycles, specifically Maragor, where the females outnumbered the males 9:1. I'm sure I'm gonna get "HERETIC!" for giving women an equal opportunity in war (it being historically "a man's pursuit" when it comes to the 40K universe), but I thought it would be interesting to see how something like this would potentially work. I'm curious to see what people think, though -- I'm not trying to start a debate about sexuality or anything like that, but I would like to know thoughts.