I already have a premise and setting I think would work well for MUSH.
The Icarus, an Apocalypse-class battleship, forever moves forward through unknown space. Its mission, to find worlds useable by the Imperium, quell native species and leave behind a beacon so that its sister ship The Osiris can come along and begin building upon the planet. A crew of just over 1 million with a volunteer regiment of 100 space marines from various legions. A generational ship that has been traveling amongst the warp for two hundred years.
You were born upon this ship, you were trained aboard this ship, and you have conquered worlds in the name of the Imperium while flying the regalia of this ship. Your brothers and sister within the Imperial Guard always by your side.
Bound for a new world, the Icarus navigates the warp when suddenly the Navigator screams out and perishes within his chambers, and the ship goes into red alert as it is swept up within a Warp Storm. The ship is wracked and assaulted, pulled out of the Warp. Heavily damaged it is caught in the gravitational pull of the planet within this system. The Icarus is lost, but the hands aboard thanks to the captain and his heroic fight with the crippled vessel are able to mostly make it to escape pods, equipment is jettisoned low in the atmosphere. You watch through the escape pod window as the Icarus plows through the planet's atmosphere and the last words of the Captain are broadcast before contact is lost "For the glory of the Emperor, Icarus shall conquer one more world with her dying breath!"