When THQ dismantled earlier this year, it effectively ended Vigil Games' Warhammer 40K MMO that had been announced a few years ago. However, Games Workshop seems determined to make a 40K MMO happen, so now there's Warhammer 40: Eternal Crusade, an MMO slated for release at the end of 2015. Judging from the website, it appears that there will at least the Space Marines, Chaos Marines, Orks, and Eldar as playable factions.
https://www.eternalcrusade.com/
Note: I updated the above link, as it had changed since my original post.