What does each faction need to un-grimdark?
Obviously this won't ever happen, but what do you think each faction would need to become "good"? (Or at least on its way to seeing "good" on the horizon). For each faction, what fundamental internal systemic or external factors are preventing it from breaking out of its own grim darkness?
Some examples:
For the Imperium, I'd suggest the religious dogmas and power systems which reward/incentivize inhumanity. Put together, these create a self-reinforcing grim darkness.
For the Necrons, I'd suggest they'd need to first get their souls back, and then second learn humility, in that order.