Brianna becomes a strong major character in her own right, from here on out. Read where they MIGHT be combining some plot lines from 2 or more books, beginning this season. I know some former characters will also make a comeback into the central story; but that's from reading the whole series (for the 2nd time) 20 years ago. Claire & Jamie of course, have their oversized share of drama.
OH and the writers are winging the whole weaving of Murtagh in, because he is so popular - about this time in the books, he is killed fighting the British, IIRC.