This is pretty much a fancy of mine, as someone who doesn't actually mind the recent changes to 40k lore too much, and as someone who thinks that there is quite a bit of potential to be had in a narrative sense from them. Writing from the viewpoint of a Primaris Intercessor squad - which requires at least 4 writers at least - I intend to explore various elements, including the partition of the galaxy, how they are viewed by others of the Imperial forces, and what it is to be the next step up in Astartes 'evolution'.
I've chosen the Unnumbered Sons AKA 'Greyshields' so that folks have a nice choice of Marines from any of the 9 loyalist legions, of Primaris who may well have been frozen during the Heresy/Scouring (probably the 'first batch' so to speak) to others that are more recent in their 'ascension'.
The timeline will be starting, as with Dark Imperium, during the closing years of the 200-year Indomitus Crusade, and possibly culminating in the formation of a new Chapter. Who knows?
As I said, this is more a fancy of mine, but if anyone is interested/wishes to join, then please say so. Any questions, ask them.
Until then, the Emperor protects!