Was thinking about the possibility of making a My Dudes Chapter.
Two things would distinguish the chapter lore-wise. First, even though they are a young chapter, they would have been blessed with the luck of investigating a space hulk very early after their Chapter's creation. The space hulk would have been an HH-era supply ship lost in the warp for millenia, filled to the brim with untainted 30k tech. Empty leviathan dreadnoughts, suits of Cataphracti armor, sicarians, etc. They would have hoarded all of that loot and put it in the vaults below their Fortress Monastery, and based their style of warfare on these relics. In game, that would be represented by them using lots of relic units and HH-era gear. Their boner for old, superior tech would be represented by them having using the Salamanders chapter tactic.
Second, soon afer their chapter's creation, they would have been tasked with eliminating a rogue AM magos, who had been involved in the creation of several SM chapters before his heretic practices became known to the Inquisition. Upon destroying him and his followers, My Dudes would have found archives written by this guy, and discovered, much to their dismayed, that they were themselves one this guy's creation. Even though they had been kept in the dark about who their primarch was, they were actually made with preserved Death Guard geneseeds as some sort of an experiment. After this revelation, just like only high ranking DA know about the Fallen, only high ranking members of My Dudes would know about this. The chapter would develop some kind of hate boner for the DG, and while their Primarch-worship turns their "Unknown primarch" into what is a thinly veiled pre-Heresy Mortarion, they make it their sacred mission to be the ones to make things right my eliminating their fallen primogenitor.
How autistic would that background be ?