So, I played WH40K way back in the day and one of the things that I loved and at this point is of only questionable canonicity is the Sensei – the sons of the Emperor.
It was the somewhat reasonable idea that for the essentially immortal Emperor ( the “New Man” or alternately Adam Kadmon if you prefer) has sired children over the ages who are also personifications of order and themselves immortal – and who also play a potential part in the saving of humanity from the Ruinous Powers.
Now, I think that it is somewhat hard to believe that the Emperor is not (or was not) aware of these individuals. I am also not sold on the idea that they are all male, that makes no sense to me, but well get there in a bit. But I can certainly buy the idea that they are essentially unknown in the current climate of the canon Imperium and that the Emperor would want to keep their existence under wraps for some arcane reason – he certainly played the long game.
So what if the stand-in for the Emperor in the Lost Imperium is one of his children? This could also be a reason to keep the Lost Imperium a more healthy and dynamic setting. No Horus Heresy means no lich-Emperor, Sensei are also immortal so this Sensei could have ruled this setting for the entire time and thus keeping the sense of the “eternal Emperor” that is important to the setting.
So that is settled, one key point for this setting is that the individual ruling it is a legitimate inheritor of the mantle (if not the title) of the Emperor – one of his children, the Sensei. What could be interesting is if we made this person female rather than male. One of the things that has always bothered me about the entire Warhammer series of games (Fantasy and 40K) is how male-centric they are – though with a genesis in 80’s era Thatcher-dominated England this is somewhat unsurprising I suppose.
But the Lost Imperium actually allows for a different take on the tropes and elements of the setting and one with hopefully a bit more balance regarding gender, etc. How exactly to make this work, along with the preserving the Imperial Cult is going to be an interesting nut to crack, but it should be fun to logic it out.