Great idea.
I don't know about what BB looks for. But what I'd look for:
1. Big, physical outside CBs with good press-man skills, good ball location skills, mental toughness, and fundamentally sound technique. Physicality and mental toughness are a must. Pure speed isn't. Draft prototypes: Kyle Fuller, Richard Sherman.
2. Safeties who are good in run and pass, with good vision and recovery time. For a bigger safety who can play more of an enforcer role, they have to have great instincts and football intelligence, and the ability to match up with TEs. Draft prototypes: Earl Thomas/Devin Mccarty for a rangy safety; Jonathan Cyprien for a bigger safety.
3. Slot CBs: good quickness and mobility (3-cone, SS time), physical, strong in run support. Draft prototypes: Brandon Boykin.