One though caught my attention pointed me to a brief from Spectral Dragon who is always good for a good rumor or two. Possible new armies we might see in warhammer 40k? Well, take a look.
Remember that rumors always require a grain of salt, but these are the types of things I love to find and talk about. Take a look at the post.
Via Spectral Dragon on warseer
the list of possible new armies I was given:
World Eaters NOTE: VERY LIKELY AT SOME POINT IN TIME.
Various other MAJOR chaos factions devoted to a single god. (All Undivided perposedly going to be represented by the Vanilla codex which I hear is next up to be started on.) I hear they want to actually do a few over the next 5 years.
Dreuger (sp?) (Squats) NOTE: NOT VERY LIKELY AT ALL, it was discussed from what I hear though. I only mention it because I know it would come up if a discussion formed.
Note that no plans are currently being formed for additional marine chapters as far as my source knows. (Outside of the ones allready released and which need to be updated, of course.)
They definitely want to do chaos factions, the rest is up in the air. I suspect they have plans to start on World Eaters.