*explodes in a mighty torrent of gore and plot bunnies, washing up around Archailfire's feet*
I had an idea for a planet that's locked in a CRAPLOAD of war between various races, and the Inquisition have scheduled an Exterminatus. The Death Ships will arrive in 24 hours.
However, something needs to be reclaimed from the planet before it's cleansed - maybe a crashed dignitary, a STC device, gene-seed, the Emperor's teddy bear, etc. The Inqusition are being stubborn and refusing to divert the Exterminatus, so a number of operatives are sent into the warzone to find the macguffin before the clock runs out.
Cue Children of Men style traipsing through various warzones and dabbling with the numerous Imperial and alien factions still on the planet.