The latest episode of Doctor Who, 'Rosa', brought back River Song's prison - the Stormcage Containment Facility. Although new showrunner Chris Chibnall is keen to create a series where each episode can stand in isolation, it seems he's not averse to adding an Easter egg or two.
'Rosa' saw the Doctor and her friends deal with one of the most subtle threats the Doctor has ever faced; a time-traveler from the 52nd century who was attempting to rewrite history. Bitterly opposed to the idea of racial equality, Krasko was a terrorist white supremacist who had resolved to prevent the Civil Rights Movement ever happening.
Related: Doctor Who: The True Story of Rosa Parks
The Doctor swiftly recognized that Krasko had been imprisoned at the Stormcage Containment Facility. Using the sonic screwdriver, she realized that he'd been considered so dangerous that the Stormcage wardens had installed a neural net to prevent him ever harming anyone. When Krasko attempted to throttle the Doctor, the neural net left him curled up on the floor in agony. He'd only been released from the Stormcage because it was believed he no longer posed a threat to anyone.
This isn't the first time we've heard of the Stormcage, though. That was actually the prison River Song was sentenced to after she "killed" the Doctor. She was supposed to serve no less than 12,000 consecutive life sentences there as punishment. Of course, River's time in the Stormcage was disrupted somewhat; she tended to break out a lot, and occasionally the Doctor would pay her a visit, effortlessly slipping past the Stormcage's defenses. In the end, River was pardoned when the Doctor erased all knowledge of himself from every database in the universe. There was no longer any evidence River had killed anyone in the first place, so she was released and her criminal record was expunged.
The interesting question is whether or not there could be a connection between River Song and Krasko; after all, both were inmates at the Stormcage. It's certainly interesting that Krasko has such detailed knowledge of time-travel technology. He even uses a vortex manipulator, the same device River used to travel through time. What's more, Krasko recognized that a battered blue police box was a possible TARDIS, suggesting he was somehow aware of the Time Lords. Could that be because he and fellow inmate River Song had chatted about her husband a few times, with River blissfully unaware of just how dangerous Krasko really was?
It's unlikely we'll ever know for sure. Krasko seems to be a typical "Villain of the Week," and it's doubtful we'll ever encounter him again. As such, this is probably best chalked up as little more than a cool Doctor Who Easter egg.