Games website Kotaku unearthed the information as part of a wider investigation of the shrinking of Hanger 13 Studios, which owners 2K had downsized following the disappointing critical reception of Mafia III in late 2016. This was despite formidable sales for the game, which shifted 5 million copies in its first couple of months on sale. Hanger 13 spent some time developing both a Mafia IV concept and also an idea for a music-based superhero game named Rhapsody, which eventually collapsed.
As part of the investigation, it was revealed that some key Hanger 13 personnel had transferred to one of 2K's other studios to work on a project code-named Parkside. According to Kotaku's article, two interesting pieces of information came out of this. First is that the studio in question is 2K Marin, the much-troubled 2K subsidiary that was effectively shuttered in 2013 following the disappointing launch of The Bureau: XCOM Declassified. The studio appears to be have been reconstituted. The second piece of information was that Parkside is really the next game in the BioShock franchise.
The BioShock franchise is one of the most revered in modern gaming, a first-person shooter series with cutting-edge visuals and intelligent (if occasionally muddled) storytelling. Created by Ken Levine and Irrational Games, the franchise was seen as a spiritual successor to the Ultima Underworld, Deus Ex and System Shock games developed by Looking Glass Studios and Ion Storm. Levine and Irrational developed the first and third games in the series, BioShock (2007) and Bioshock Infinite (2013), whilst 2K Marin worked on BioShock 2 (2010).
After the release of BioShock Infinite, Levine felt burned out from making high-pressure games with budgets in the tens of millions of dollars. He wanted to make smaller-scale, narrative-focused games. To this end Irrational was rebranded Ghost Story Games and downsized massively. Levine and Ghost Story have been working on their debut title ever since. Given that the first 3 games had sold over 25 million copies between them, 2K confirmed that the BioShock series would continue, but some commentators were dubious of the series moving forward without Levine's guidance.
Nothing is known of the next BioShock game save that it will have some big shoes to fill without Ken Levine's singular vision. However, given that BioShock 2 was also made without any involvement from Levine and was an extremely strong game, that's not perhaps as much of an issue as it could have been.
The game is likely a long way off still, given it's not even been officially announced yet.