They made 3 clones of her, portman could play triplet clones in the new movie...
They could have her play an adult Jaina Solo (Han & Leia's daughter).
Base the movie in the Legacy of the Force era. Have Harrison Ford, Carrie Fisher and Mark Hamill return as their characters. The series is 40 years after ROTJ, so it wouldn't be a problem. Have the focus be on the "new generation" being brought up by the trio and the stories of Anakin, Jacen, Jaina & Ben. Since the original 3 actors are already pretty old now, and wouldn't be doing much in the way of action sequences (this could be fixed with CGI and computer models if needed) it wouldn't be a problem. You could get 4 up and coming actors (2 would have to continue with their characters) and actresses.
Just call the new series, "Star Wars: Legacy"
Episode 1 would be Legacy of the Force
Episode 2 would be Legacy of Vader
Episode 3 would be Legacy of the Jedi
The first would be focused on an introduction and catchup, the second would be on the turn of an important character to the dark side, the third would lead to the final "conclusion" and consequences of the previous 2.
It could set up animated series between Legacy & ROTJ, and movies taking place during and after legacy.
They could continue with movies after these are finished and in subsequent productions just have Ford, Fisher & Hamill do short cameos or voice-over work, with the majority of the action focusing on their children.
The downside is they'd have to do a lot of catchup explaining the absence of Chewbacca, Mara Jade (Luke's wife) and important planets in the universe.
The other downside is 2 out of those 4 actors wouldn't be able to continue with their characters, and the series is also pretty dark at times, with a lot of pain and suffering going on.