I agree in not understanding this forum's love for the Eldar. I mean they're neat and all but they're nothing compared to Orks (and I don't care what people say, Orks are quintessentially WarHammer 40k from its earliest roots, heavy metal songs and all). There's a lot more groups I'd like to explore in RPG form before the Eldar, even the Tau because that opens the door for their different brand of politics (One could argue that a game about Tau explorers would be as close to a 40k game of Star Trek as you can get)
That said, I agree that Necromunda would be better suited as just a splatbook of sorts UNLESS it's more than just the gangers. I do think there's interest in playing a game of Imperial citizens on a home world that could allow either for crime sprees or playing the side of the Law. It would essentially be a return of GW's Judge Dredd RPG where you could play both sides of the coin and they each had different tones of gameplay. It could either be about sneaking around, skullduggery, and plotting big crimes while fending off rival gangs, or it could be investigation, pursuit, and enforcing a very harsh form of justice. The game could also feature a very tongue-in-cheek humor that stems from the absolutely rediculous totalitarian society of the Imperium, almost along the lines of a Paranoia style game. I know humor is a big no-no around these parts and everyone wants their game to be super serious and grim but certainly one of the best parts of the WFRP game from which this series gains its heritage from was the soft pokes at its own expense, to show that it didn't take the game's rediculous setting too seriously.
If you were to look in the same vein in that each RPG game has its own unique core mechanic that changes the flavor of the game entirely I would actually like to see a game that specifically revolves around large fleets of the Imperial Navy. Basically make an RPG where it's not measured by your individual charcter, but by the composition of your strike force. Players would have to work together to create complex combat actions and fight large scale galactic wars above beyond those found in Rogue Trader. But that's just me.