Crackdown lead designer Billy Thomson and other key members have set up shop in Dundee, Scotland, and rumors indicate a Crackdown sequel might be their first project.
The first Crackdown title was developed by Realtime Worlds, which is now developing its cops vs. robbers PC MMO titled APB. Microsoft apparently owns the Crackdown property, which would lend credence to the story of a Ruffian-developed sequel.
Gamasutra also cites a recent Game Informer article as evidence that Crackdown 2 is in the works. The article stated: "[Realtime Worlds] is currently working on APB and not Crackdown 2. The sequel is in production with a different development team."