While the Guard are far and away my favorite faction, I'd love to see Sisters of Battle gear added. They always seem to get forgotten among the Spees Marines and company, but they scratch that particular itch that's "burn everything with fire, then burn the ashes."

Sisters of Battle gear would be something I want to see as well. And the Commissar's glorious long coat, to help inspire the troops by fear and example.

Skull probe gremlins?
Oh, also, if it's possible, having the goggles and mask combined to take up a single slot as an option would be great, since currently you have to pick between goggles or the jumppack from the other helmet mod.

Second Skull probes, though I realize that would be waaayy down the line lol.
Other than that, Inquisitor gear(pretty much just your average sci-fi steampunk), pskyers doodads, scriptures nailed to the face. Different armour sets to go along with those and why not throw some mechanicus influence in there. Cables and various fleshgrafts or enhancements, that sort of stuff wouldn't even be strictly 40k.

Or maybe some prosthetics for the prostethic mod. New models for 40k grenades. Assassinorum gear...WRRRRRYYYYYYY :P

mmmkay, i'll keep those in mind. a lot of this stuff is more me not being lazy and actually doing stuff, and having to learn new parts. not sure how viable the skull probe thing would be, it sounds kickass and i'm sure it's possible but lord knows i'd have to figure out how to mess with the gremlins first