Just bought The Koronus Beastiary and was a bit disappointed it didn't include some guidelines for creating advanced Xenos races, just primitive cultures. Maybe the writers felt there were simply too many possibilities to come up with a proper selection of ideas? Or is there actually a generator somewhere else in the splatbooks (maybe an Ordo Xenos guide for Deathwatch?) I'm missing?

One possibility might be to create the primitive cultures as per the Beastiary and then "advance" them from there... I'm also kind of leaning towards imagining what the original species that "devolved" into the bat-like Species X.101MK2 (or somesuch, don't have the book to hand) on Orn was like, since they had a ship "as big as a War-Sphere".

Or has anyone come up with a guide?

Just my tuppenn'orth but I think there are enough advanced races as it is; if every system and cluster is populated by a space-faring civilization you quickly cross from 40k to Star Trek.

In the Expanse alone there are already humans, Orks, Eldar, Tau, Rak'Gol and Stryxis criss-crossing the stars, plus the remains of the Yu'Vath and Egarians, plus the semi-ignorant Zaythians(? - the ones with the giant moving cities, anyway); just how common is sentience in your version of the 40kU?

If that's what you want, however, I suggest ripping off and recombining societies and civilizations from other media - tv, films, books, games - and history itself. What works for most sci-fi writers can work for you

Just my tuppenn'orth but I think there are enough advanced races as it is; if every system and cluster is populated by a space-faring civilization you quickly cross from 40k to Star Trek.

In the Expanse alone there are already humans, Orks, Eldar, Tau, Rak'Gol and Stryxis criss-crossing the stars, plus the remains of the Yu'Vath and Egarians, plus the semi-ignorant Zaythians(? - the ones with the giant moving cities, anyway); just how common is sentience in your version of the 40kU?

If that's what you want, however, I suggest ripping off and recombining societies and civilizations from other media - tv, films, books, games - and history itself. What works for most sci-fi writers can work for you

Well, true about there being lots of races, but specifically I want a "mystery" species that the players can unravel. This species was responsible for the dereliction of the RTs vessel prior to him finding and recovering it. There's no record of such a species in Imperial databanks, but they most likely came from the Expanse. Are they a threat, or was the initial attack self-defence against a Puritan "Xenos Must Die!" naval captain, in which case they might be exploitable?

Hmm, interesting. You could check out http://www.seventhsanctum.com/generate.php?Genname=alienrace and http://www.scifiideas.com/alien-species-generator/ - the first is more detailed - or, if it was me, I'd decide which of the options you give (threat/exploitable) best fitted the themes of my game and work backwards from there.

If you want to progress a group using existing supplements you could create the Xenos race with The Koronus Bestiary or Stars of Inequity, then use the Nemesis Path in Hostile Acquisitions to advance them. Use more Nemesis Points for higher ups and less Nemesis Points for more basic troops, and don't worry too much about Profit Factor. It's a bit clunky but it would work to produce some degree of advancement with the current books.