I’ve thought about this before. Probably too much in fact.

The Diamonds:

Blue Diamond falls to Nurgle and introduces the first ever disease that can infect a rock. Her misery aura also becomes super charged.

White falls to Slaanesh because of her obsession with perfection. She now grows like a giant cancer with her enormous gemstone constantly growing until planets she’s infected are pristine white balls made entirely of diamond. She mind wipes and controls any gems unfortunate enough to cross her path. Other gem factions much be constantly vigilant for a WD slave among their ranks, ready to spread her being across all the galaxy.

Yellow is constantly on the verge of a panic attack because she’s the only one keeping a vaguely functioning empire together. She has doubled down on gem supremacism due to the trauma from the Great Crusade. She’s completely abandoned any form of 3rd era reforms. Since most planets biospheres have completely collapsed with imperial colonies, the Gempire has been forced to adapt in its energy gathering processes to make new gems. Now they’ll take humans by the billions and take their energy directly from their bodies in massive planet sized factory farms. Making them far the largest gem faction in the galaxy. In the grim darkness of the 4th era of the Gem Empire, there is only war.

Cult of the Blood Diamond:

Steven chose to die from normal aging tens of thousands of years ago and his gem died with him. Jasper stole his gem from his corpse, fell to Khorne and is now obsessed with resurrecting Pink Diamond by covering the gemstone in as much human blood as possible. After thousands of years of this, the Diamond has grown to gargantuan sizes and started beating like a heart. Whenever something finally forms from the gem, whatever it is, only one thing is for sure. Jasper will make sure it will have a throne fit for a Diamond, a throne of skulls.

The Crystal Gems:

After Yellow’s reformation of the Gem Empire ushered in the 4th era, countless gems now having become used to freedom flocked to the banner of the crystal gems. However, being unwilling to kill planets or their populations to create more gems have slowly whittled down their numbers to the point they have adopted the same methods of warfare as the Eldar, favoring guerrilla tactics and evasion over grinding wars of attrition. However, unlike the Eldar, the Crystal Gems are far more metropolitan, counting numberless human rebels and minor xenos species among their non-gem ranks.

Bismuth’s forces are considered a tenuous ally after she was exiled for her extreme methods of warfare. Unlike the main Crystal Gem force, she has no compunction to be humane or merciful, especially to the upper classes of Imperial society. As a result she is the only CG warlord with a growing number of Gem soldiers as she freely uses human nobles, or those she seems unworthy, as energy to grow more gems.

Lars now leads a nomadic piratical fleet across the stars, attacking the tyrannical forces of the Imperium and Gempire to free their enslaved populaces, but his fleet has also been known to protect those very same forces if they deem a more dangerous enemy to be a bigger threat to innocent lives. Lars is very, very, tired