Abbie Cornish might make the perfect "adult Newt" for this. To me she looks a little like Carrie Henn. That is if Newt is even in this film. Also, depends on how old Newt would need to be in this. Cornish is 32. That seems about right, unless this new film is set 30 years after Aliens (we're coming up on the 30th Anniversary of the film next year). In that case, Newt would be pushing 40.
The thing I like about Abbie Cornish is she is pretty enough for a big budget Hollywood movie like this, but is not sporting supermodel looks. She seems like a pretty woman one might see in everyday life. She seems believable in the Alienverse.
Not to downplay her looks, as she is very attractive IMO
Sigourney as Ripley rocked the Amazon warrior stature. Cornish would work as a slightly stockier action heroine.