On Bryce Dallas Howard's role-
Early versions of the script had Marcus as the central character. With a much smaller role for John Conner. Setting him up for more screen time in the sequel as the leader of the Resistance. All of which changed when Christan Bale was cast in the role.
While John's scenes were increased Kate's limited presence seems to be a hold over from that earlier incarnation.
I had read something similar about Connor not being the main character in an earlier version of the story. But despite the screentime Bale got, the writers didn't do a good job making Connor the main character anyway. It still very much felt like Marcus's story, with a beginning (of sorts), a middle, and an ending. I think they should've stuck with the original idea of keeping Marcus the main character, and maybe just cast Worthington as Connor and Bale as Marcus if Bale to give him the appropriate amount of screentime befitting his star status.
Bale as Marcus would have been a NIGHTMARE. The only reason this film is as good as it is is due to Sam Worthington's performance as Marcus. Bale would have grunted through the entire role. The movie wouldn't have been nearly as effective and Bale would have continued his descent into living joke status with that stupid fucking grunt of his.
John Connor's role must have really been small in the original. Despite the screen time he's given, he takes a backseat to Marcus. Bale does well from a action and physical standpoint but his character doesn't exactly comes across as one that viewers will embrace.