This news shouldn't be all too surprising if you followed any of the Thor 2 news.
According to the Hollywood trade sites, Chris Hemsworth's Thor will have a new love interest in Thor: Ragnarok, played by Creed star Tessa Thompson.
The character is presently unknown, but Deadline says the character "will be kind of a superhero" and will appear in other Marvel movies.
THR also has it that Jaimie Alexander is returning as Sif for Thor: Ragnarok and that filming starts in June.
Regarding Natalie Portman, it was said a while back she wasn't happy with Marvel over the ouster of Patty Jenkins from directing Thor: The Dark World, and that Portman was forced to return for the movie that she didn't originally want to be a part of, but only agreed when Jenkins came on board. Obviously when things went south between Jenkins and Marvel over Thor 2, things went further south between Portman and Marvel. There is also the fact that Thor: Ragnarok looks to be set away from Earth and more in space, so obviously Portman's Jane Foster wouldn't come along for the ride.
Thor: Ragnarok has a November 3, 2017 release directed by Taika Waititi also starring Tom Hiddleston as Loki, Mark Ruffalo as Bruce Banner/Hulk and Cate Blanchett is the rumored villain (possibly Hela).