I propose a dark, post-war AU:
In the final battle against Haggar, the witch kills Pidge’s entire family right in front of her. She hits Voltron with such corrupted energy that it shatters the superweapon. It’s only together, with just their bayards, that the Paladins finally defeat her. Voltron is gone. Haggar is gone. The universe is finally at peace, and the Paladins are there to help Allura rebuild it.
Things go smoothly for a while. Allura marries Lance and together with Hunk and Romelle (who also get married), they attempt to rebuild the Altean community and bring the Alteans that Haggar held captive back to good help. Through an unexpected chain of events, Keith finds out that Axca is his half-sister from before Krolia met his dad. Her father was a Blade that fell in the war long before Keith was born. Keith and Axca work closely with the Blade and with Nyma and Shay, now the leaders of the groups that were formerly the Voltron Coallition and the Intergalactic Rebellion and happily married to each other. Keith and Matt were going to get married when the war ended, but they never told anyone. No one knows what Keith lost in that final battle. Not even his sister and mother.
Shiro and Pidge always had feelings for each other, but they never acted on it. Shiro always worried that he wasn’t good enough for Pidge, and Pidge thought Shiro would reject her for being younger (despite her being an adult). He wanted to ask her out after the war was over, but when her family died he knew he couldn’t. She grew cold and distant from everyone, hiding her feelings behind a fake smile. He couldn’t put any more pressure on her. So he simply supported her instead. Together, Shiro now the leader of the Intergalatic Militia and Pidge head of the Technologies Restoration Unit, they worked to undo much of the damage the Galra Empire had done under Haggar and Zarkon’s rule.
Several years pass by and no one notices how the color slowly fades from Pidge’s face. How her eyes lose their lustrous gold tint and gain an eerie yellow look to them, almost glowing in the dark. They don’t see the long purple marks that form on her back, as if a monstrous animal clawed its way down her skin. And they certainly never see what she works on all alone in her lab.
On the third year anniversary of her family’s death at the Battle at the Universe’s End, communications across the universe simply cease. Panic spreads like a wildfire and chaos ensues as the Intergalactic Military scrambles to maintain order. But one by one, their bases are locked down. Every major center of activity in the universe is simultaneously captured. Every single Altean (with the exception of Romelle, Allura, and Coran) turns on their fellow citizens of the universe, acting as generals for the invading forces. Sentry drones with a design unlike any that’s been seen carry out orders from a mysterious source. No one knows who has taken over. No one knows why.
And then the hunting begins. Blade members fall. Old Rebellion generals are taken captive. An order is intercepted just in time to save the lives of the former Paladins. It instructs the invading force to kill every last member of Voltron and it’s close associates on sight. Once heroes of the galaxy, they are in hiding. And they are separated from each other without any form of communication.
Months pass and Shiro finds himself slipping from planet to planet, organizing a loose resistance and searching for Pidge. No one has heard from her since the Blackout. He was the last to see her, just before she departed to Olkarion. He fears he will never see her again.
But what he doesn’t know - what no one knows - is that at the heart of the intergalactic takeover is the former paladin of the green lion. With the resources of the universe at her disposal, and the corrupted quintessence flowing through her veins, she believes she has the power to find a way to bring her family back to life. But in order to do so, she needs to reshape the universe to her design. She needs to play god. And no one can stand in her way. No one.