Aliens in 40k aren't Star Trek aliens. They aren't humans with some weird **** on their foreheads. They are true aliens, and that means that their psychology is very different from ours. 40k aliens do not make good characters for a roleplaying game in my opinion.
This is another one of those topics that are going to polarise the hobbyist community as much as Marines... or Eldar.
On my behalf? I disagree with you. Not disrespectfully, though. What I would rather suggest is that the 40k aliens aren't really that different from the Star Trek ones, though, beyond the fact that I'm sure that Gene Roddenberry is not kicking himself (err, from the grave) about not hiring GW artists and writers (believe it or not). They do a very good job of creating some cool imagery and writing some text that vaguely talks about how alien they are without actually telling you how alienor not alienthey actually are.
If you look at the literature, though, it's not about being truly alien but being believably alien. It often seems about being presented with what is described as an alien culture, and then being able to see familiarity in itonly to be baffled when they do something crazy (as determined by the plot). The thing is, you can find examples of this kind of culture in the real world if you're willing to look at the type of norms that might be defined as Western gaming culture. Delve into anthropology. You will see how human beings with a whole slew of different mores, ideologies, metaphysical paradigms etc., will be as alien to you as a... well, whatever race you to see that is attempted to written up as anything other than a plot device.
As a plot device aliens can be "alien" because you only have to justify whatever crazy action they're going to make. As a PC race, they just have to believably described such that the PC can have some cultural context that extends beyond just a body to be placed in the cross-hairs. Even though foreknowledge is kind of expected, we see this kind of "cross-hair" alien in Creatures Anathema and, forgive me, perhaps even things like Lure of the Expanse.
As always, though, YMMV.
I think FFG could do far worse then the release a "Core Core Book" the biggest flaw or obstacle would be careers, as each "game" thus far has totally different careers.
Couldn't this be achieved with just playing a mathematical game? Different "tier" points working on a different ratio? Or just refer to the power levels as something discrete rather than encompassed by a different system? x,000 points for Dark Heresy, 10,000-n,000 points for Rogue Trader, etc.
However, a few generics would work (as I said a few years back when DH first came out, the D20 modern basic careers of Strong, Smart, Tough, Fast and so forth careers) with sub notes of how each of these new careers work in each game (for example, as rank 1"alternate career ranks" for DH, as part of the origin path for RT and whatever for however DW works).
That's one of the interesting things that I liked about skrittibak's (sp.) interpretation of 40k RPG. Even more flexible than Dark Heresy Career Paths even though they are inherently based upon the same mechanic. (It's where, for me, Dark Heresy gets into some interesting narrativism.)
After that 40K Core Sourcebooks. The Xeno Codex with critters, gear, adventures, worlds and info that work for all three games. The Imperial Armoury full of gear for all three games. The Index Calixis as a setting guide and so forth.
I think that you might have Adam France drooling over you and wondering if you want to propose now, Peacekeeper_b!
Sorry, while not really well written, having finished the Kal Jerico trilogy of Necromunda Novels, it just sounds like a good 40K setting. Campy, sci-fi, fun.
Is there a way of talking about such things generically? After all, hiveworlds have some shared imagery even before you go beyond the "every world is unique" thing as applied.