Sequel or Spin-off?
I haven't seen the movie yet but reading these posts has me worried that this is not a sequal but a spin-off.
The differences are simple:
A sequel ties together or answers questions from the previous movie.
A spin-off just takes 1 or more characters from the original & has a completely new/different storyline with no relevance to the previous movie.
I thought BI2 would reveal what we all suspected from the first that she was the killer (when Nick reads the print-out of the "Shooter" it describes exactly how Gus dies in the elevator).