Off topic point; a 40k survival horror game would be perfect. I personally vote for Warp shenanigans on a ship w/o a Geller Field.
If that is a ship in Warp completly without Gellar Field, it wouldn't be survival horror. It would be /d/ worthy Hentai.
Comon back me up imperium
For the Emperoooooooor!Even imperium has more opportunities for rising up through the ranks then that caste system. (when imperium surpasses you in something social or economical, something must be really wrong with your system)
It is known that any (literally any, from underhive scum, through gray factory workers to highest nobles) human citizen of Imperium can become Inquisitor, for instance. You just need to be the right person. Any Guardsman can, in the end, become even Lord Commander Solar, he just need to prove not only his tactical abilities (Before you go "Hurr Chenkov Durr, remember Macharius) but also his balls, by somehow surviving the grim and dark battlefields of 41st Millenium. Regular human boys are the ones who become future Space Marines, be it boys from feral world or Terra itself, and every Space Marine can ultimately become Chapter Master of his own Chapter. Imperial Navy, well, it's harder, but once you made it to Petty Officers and the ship you're on doesn't explode, you're set (also, contrary to popular belief, most of crew on Imperial Navy spaceships are actually volunteers since they pay is much better and conditions are better than on many Hive Worlds) Commissars, Sororitas, Stormtroopers, high-level clerks of Munitorium and adepts of the Administratum (laugh if you must but remember that Grendel (the level 2 Adept who one-shotted a Charnel Deamon... with a knife) is now canon, not to mention that they proably have the best lives around if you're content with 20 hour workday) all come from Scholas Progenium and are orphans, for fucks sake. Anyone can rise to the top in Imperium, the road is ridiculously grimdark hard and requires a lot of luck (or Emperor's watching over you), but it's possible... meanwhile Tau...
Tau have caste system. Fuck your ambition.
Also, their ranks for Humans and other auxiliaries stop at an equivalent of sergeant, IIRC.
Well, they do have an entire caste devoted to diplomacy
Pretty sure they'd cooexist with the Imperium if the Imperium had any interest in that whatsoever, or even the guile to lie about it (like the dark eldar).
That's not what's happening. Their "diplomacy caste" exists to convert other races to "Greater Good" or presenting ultimatums. They're the aggresive side here (there were like two acts of agression on Imperial side total, and that was just because they wanted to reclaim what Tau took) and considering that they're on total backwater and Imperium is very busy with important enemies, if Tau actually wanted a peace on equal terms instead of "ACCEPT GREATER GOOD OR DIE, GUE'LA", the Imperium would be more than happy to let them exist, for now. To be honest, Tau proably don't even realize how ridiculously big is Imperium when compared to them or they would be more talkative.