To answer the OP's question, I have another question; at what point in the Imperium's history do you want to use for your start?
Do you want to play at the time where the Great Crusade was just beginning to push off Terra, under the command of the Emperor (and his shadow, Malcador)? What about during the height of the Age of Apostasy, when the High Lords were usurped by the Eclesiarchy, led by the ruthless and paranoid Goge Vandire? How about playing as the government of Ultima Segmentum during the Damocles Crusade, when despite their xenophobic tendencies, the Imperium (especially the Astartes Chapters assigned to the crusade) was able to see that xenos, in this case the Tau, were capable of honor? Or do you want to play during the Time of Ending, just before Cadia fell and this "Gathering Storm" nonsense kicked off?
For the above examples, here is what I would recommend
- Great Crusade: Militarist, collectivist, xenophobe, Despotic Autocracy (or whatever the non-ethos autocratic government is called). The new Imperium may not have been fully united, especially near the end of the crusade, but the majority of the population was (on the surface) behind the Emperor.
- Age of Apostasy: Fanatic spiritualist, xenophobe/militarist (your choice), Divine Mandate. This was the time that the Eclesiarchy was truly ascendant. Such was their power that when the High Lords tried to put some form of control on them, they simply left Terra and headed to Ophelia VII. The church was both very xenophobic, and highly militarized (the siege to retake the Emperor's palace only ended when the captain of the Cusdotes convinced Alica Dominica, head of the Brides of the Emperor (Vandire's bodyguard) to turn on the Eclesiarch)
- Damocles Gulf: Fanatic Militarist, collectivist, Military Junta. During this period, the Imperium committed far more forces than they normally would to deal with an upstart xenos race (Tau) than they normally would, hence the fanatic militarist. In addition, as the campaign dragged on, many in the Imperial leadership, including the space marines, began to develope a grudging respect for the Tau, hence the lack of xenophobia for this start, though don't kid yourself, the Imperium would have purged the Tau in the end if the Tyranids hadn't showed up.
- Time of Ending: Militarist, Spiritualist/Collectivist, Xenophobe, Theocratic Oligarchy/Military Junta. Perhaps the main time period that people think of when they hear the words; Imperium of Man. Still militarized against threats from all sides, still enslaving most of the population, still devoted to their corpse emper... Er I mean living god, still suffering not the xeno to live. I suggest using an oligarchic form of government, as the Emperor himself doesn't really have a say in what happens anymore. Pretty much every order comes from the High Lords, the oligarchicist oligarchy that ever oligarchied.
Regardless of what you pick, psionics need to be kept to a minimum, but stay in character. Psi jump drive is okay (into the warp you go, hope you don't summon daemons/unbidden) but full psi armies is definitely heresy, you are not Thousand Sons. Also, No AI (Abominable Intelligence), the Ad Mech would agree with me. Slave armies can work as penal legions, but just sticking Commisars in all your assault armies is lore accurate as well. Stay away from xenomorph armies but clones can represent your Death Korps of Krieg. Don't splurge on gene warriors, there aren't that many space marines, though if anyone gets psi warriors, it's them.
As for your starting FTL, none of them really fit (warp is too rooted in reality, hyperlanes are too eldar (webway) and wormholes don't fit the lore) but if you had to choose one, warp.
Huh, that turned into a wall of text, sorry about that. Hope it helped though. Just remember, that the Greater Good is bet... *BLAM*
THIS AUTHOR HAS BEEN EXECUTED FOR HERESY. ANOTHER AUTHOR HAS BEEN REQUESTED FOR FROM CADIA CENTRAL RECRUITING COMMAND. PLEASE HAVE A PRODUCTIVE DAY, LOYAL CITIZENS
