Generally - even though we love characters and want great stories to never end lol - I don't think its ever a good idea to carry on after a series/trilogy has ended.
And this one in particular ended so well. Yes on the growth of the characters - I just love that Nat does the right thing in the end and dies a hero. I especially love him finally showing his affection/love for Bartimaeus - first when he sent him back to the Other Place to recover and then at the end to Dismiss him.
I think it was pretty brave and unique of Stroud to make Nat, a main character, so progressively unlikeable (but I always enjoyed reading his sections of narrative). In contrast, Kitty at times was a little dull. Its interesting to me that in many ways, she was no better then him. They both had pre-conceived prejudices about commoners, magicians, and demons, which they both learn to look past. I was surprised at the end she chose not to help rebuild the government. That seemed wrong to me, just because they needed someone there who understood and cared about the treatment of everyone, especially demons.
I really want to read the prequel - but I also kind of don't! Bartimaeus without Nat!??!! Me dislike.