It's already being developed. Going back years ago they mentioned it as Warhammer Fantasy was under development, along with other games like Age Of Conan, and the aborted Gods and Heroes.
Generally speaking Warhammer 40k seems to be a bigger liscence than Fantasy is nowadays, and I am hoping they take more time with it. While Warhammer Fantasy had some interesting ideas at work it was simply put a mess. The characters all looked the same, more so than in say World Of Warcraft where at least the gearsets have more diversity so everyone winds up looking fairly differant for all practical purposes. Telling a priest from a Warlock from a mage for example isn't always 100% obvious while in Warhammer everything was so cookie cutter you could immediatly tell someone's class just by looking at them.
On top of that there were huge game balance issues, and of course a complete blindness to the whole "evil chic" dilemma that seems likely to destroy KoTRO before it's actual launch.
The best way they could approach Warhammer 40k is to take an approach similar to the "Dark Heresy" PnP RPG. That is to say keep everything Imperium based PvE for the most part and have the characters set as adventurer types. Inquisitors, Active Duty 'leet soldiers, Space Marines, etc.. all being NPC material. Then focusing on the setting itself. Generally speaking concepts like Space Marines are hard to work around for PCs in an RPG because honestly they just sit around intentionally cloistered until it's time for them to kill something, then they go back to being cloistered. They generally do not go beebopping
around the galaxy solo looking to see how much trouble they can get in. Inquisitors do, but they aren't something that can be balanced without effectively making everyone say an Inquisitor, or Rogue Trader (which can be almost as bad as an Inquisitor at the high end).
The bad guys in Warhammer 40k don't really cooperate for the most part, or at least not willingly (except for orc cannon fodder) and honestly it's hard to conceptually balance. The idea behind Warhammer 40k as a minatures game was that the individual units of some armies were simply better than others, but the better a unit was, the more points it cost. Thus for an equivilent number of points Orks or say Imperial Guard would wind up outnumbering a group of Chaos Space Marine. By the same token the more practically useful things like chaos mutations are the higher the point value of the piece.
You just can't make this work in an RPG while staying true to the concept without causing some god awful game balance issues.
What's more I'll be honest in saying that watching everyone run around in identical Space Marine armor (except for color patterns) simply wouldn't excite me with the diversity of the game and character options.
There are various reasons for imperium factions to fight each other (the minatures game was set up so any army could fight any other) so I'd imagine they should set up PVP without having playable "bad guys". That also gets around the issue of trying to balance Necrons.
Honestly Necrons, 'Nids, Genestealers, etc... are more or less mindless on an individual level as I understand them. I just don't see how you could have a Necron player character or how it would coordinate with others. I mean unless your talking a Tomb Lord or something and then the character would be way too powerful due to it's legions. What's more it wouldn't work eith anyone, not only does it want everyone dead, but nobody would be STUPID enough to trust it if it tried to implement some obtuse "oh well, we're misunderstood, we can be friends" strategy to weaken and betray another group at a critical moment.
At any rate, we'll see what happens, but I'm pretty sure I've already heard that this game is being worked on and will be coming. Very, very hard thing to implement.
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