I am still in the very early stages of the concept, however Iam looking for a very Greco/Roman ideal, without going down the road of the Ultramarines. I really like the idea of following the historical/Mythalogical aspects of Aeneas, but in the 40000 setting, ie making the chapter heroic fugitives and the last of their society that havent been enslaved or destroyed, and from then going onwards to greatness.
Iam thinking that they are from a world that had decended into civil war, with their antagonists being Slaneeshi cultists, prior to being defeated a relatively small contingent escape to a nearby system, and settle. Shortly after a fleet based Space Marine chapter (Black Templars) encouters and brings them willingly into the imperium. So impressed with the martial prowess and natural disipline of the population, they petition the high lords for the creation of a new Chapter.
I know I know, loads of holes, and very little detail __________________
But its a start