This is one of those "out of left field" ideas. However, it should be noted that Sandra Bullock did work with director Paul Feig on the female buddy cop comedy "The Heat" which was released in 2013. Bullock is a pretty talented actress, and capable of handling both comedic and dramatic roles. Ghostbusters always walked a pretty fine line between comedy and drama, and in order for the concept to work, anyone in the franchise needs to be able to sell the reality of the situation. We need to believe that the spooks and spectres are real. Enter Bullock. She could easily be the Venkman of the group.