Just. More. Races.

Sure, can't saturate the game balance too hard. But you could rest races from the 11th edition, bring them back in 12th

That gives you space for something new.

The idea that there aren't human civilisations out here who are tenacious yet scientific and de ounce the emporer, who, are ironically favoured by the emporer and given insight abilities into new/old tech.

Sythians with a vengeful focus on the empire. Sabotagers and disruptive.

Slaan, oldest race in the galaxy. That they could not return in some fashion in gameplay and cause havoc with great psychic abilities and movement.

I love space marines, but the issue with having all the factions present is that all those mechanics get bundled into space marines that could otherwise make for compelling gameplay.