Natalie Portman is "Done" in the Marvel Universe
Natalie Portman has not been mentioned as Thor: Ragnarok gets well and truly into filming, with Tessa Thompson cast as Valkyrie for the new film with the believe that she would be a new love interest for Thor.
"As far as I know, I'm done," Portman said in an interview with The Wall Street Journal. "I don't know if may one day they'll ask for an 'Avengers 7' or whatever, I have no idea. But as far as I know, I'm done."
Portman appeared in both Thor films so far, and has been referenced in both Avengers films so far. Whether or not this means the character Jane Foster won't be returning is a completely different question, and one which certainly doesn't have an answer.