…and a whole book about dusty old coots who read alot and likes to visit ancient Xeno civilisations before going mad form what they learned? No thank you.
Adpets have always been a tough sell to most players (there is a thread on that very topic currently in the main DH Forum section); an Alternate Rank that makes them substantially mose useful to an Ordo Xenos team can only help, i.m.o.
Xeno hunter career? They're called Imperial Guardsmen in the Imperium.
Standard Guardsmen kill xenos the same way they kill everything else: through overwhelming numbers. But the fluff has always made references to elite xeno-killing specialists with skills beyond those of rank-and-file Guardsmen; the tabletop game made this fluff part of the rules in 4th Edition, which added a Xeno-Fighter Doctrine option to I.G. armies.
…besides…what is there to have?
Off the top of my head, in addition to a Xenoarchaeologist and Xenofighter, and a xenotech specialist like you mentioned, I could see some kind of Diplomat option that negotiates with other cultures, including xenos (yes, it does happen in the Imperium, despite what the Adeptus tells the unwashed masses…), plus a buch of Radical options: humans raised by xenos (a la Stranger In A strange Land), mad scientists who illegaly graft xeno organs and limbs onto humans, and some kind of Cold Trade specialist…
Plus, more fluff: info on the Calixis Deathwatch, more details about the Yu'Vath, the former rulers of Dread Calix (including examples of their ruins and technology), and additional Cold Trade material, ideally including a sample cartel with key npcs.
Crunch-wise, can we ever have too many examples of xenotech? Weapons, computers, vehicles, drugs, toxins, viruses (both organic and cogitator), specially bread guard/servant beasts, etc.
Rules for including a Deathwatch Killmarine in an Ascention-level party for those who don't also play Deathwatch would be appropriate.
The core, though, would have to be more xeno stats! The Radical's Handbook mentions the Calixis-specific Morgauth ('scavenging reptilids'), Xenarch ('of the Northern Rim'), Draethi ('dark alien race'), Q'Orl ('a particularly insidous insection race'), Fra'al ('merciless, highly psychic'), plus a mercenary army called the Cortax (I assume it's a group of various races, and not the name of a destinct race in itself); I'm not a big fan of repeating the same creatures between different WH40KRP books, but I wouldn't mind combining stats for Loxatl mercenaries (from Mark of the Xenos), Sslyth mercenaries (from Koronus Bestiary), and Kroot mercenaries (from various Rogue Trader and Deathwatch sources) in one volume. -And Hrud, with their focus on stealth and warp-based technology, would be a good fit for DH.
And of course, space permiting, a module. Not everyone likes the practice of including mini-modules in Sourcebooks, but, since nearly every DH module focuses on fighting heretics and/or daemons (Maggots In The Meat is the only exception I can think of), I think a xeno-focused module would be a welcome novelty.
Tell me with a straight face that you wouldn't buy this book…?