A conventional start with 2 NPRs who shall represent the Tau and greenskins, other races encountered may include the Eldar, Necrons and Tyranids.
, NPR's will be non-TN initially except for the 5 races above, they represents the scattered barbaric human worlds and higher tech martian forge worlds which need reconquering.
Additionally there is a neutral population of 10 billion humans on an earth which has been ravaged by war, intense radiation and dust has reduced the world to negative population growth, but the Imperium has enough underground infrastructure to protect its small core population.
Mars is colonized but retains it's hostile environment, underground infrastructure provides the home for 100 million cultists who primarily man the great commercial shipyards which once colonized the galaxy, advanced industry and research centers of the Mechanicus. Resource wise it is poor having depleted much of its reserves during the great exodus.
Earth has not much to show for it except for it's population and scattered conventional industry, as well as massive reserves of low tech soldiers and a small core of highly skilled marines, however it does possess considerable mineral wealth.
Luna resists your control, but if it's hardened PCD's can be conquered you can put it's genetic technology and ground force training centers to good use, however rumors persist that the orbiting ruins of it's naval shipyards are still operable.