Even though rumors persisted for a few months, Take-Two Interactive officially confirmed that BioShock 2 is in the works due for release in the fourth quarter of fiscal 2009.
In their first quarter fiscal 2008 financial results, Take-Two reported that newly formed studio 2K Marin will handle BioShock 2’s development.
2K Marin is primarily made of up ex-BioShock team members and will focus on creating games for 2K Games’ IPs. A follow-up conference call revealed that 2K Boston studio founder and BioShock lead designer Ken Levine will be involved "in some way with the development of BioShock 2."
Lastly, Take-Two mentioned that BioShock has since sold over 2 million units worldwide since its release in August.
I certainly enjoyed BioShock so I would enjoy more of the same; I think the story is pretty dynamic to branch-off with.