According to The Hollywood Reporter, two actresses are slated for screen tests: Emilia Clarke (Daenerys Targaryen in the HBO series Game of Thrones) and Brie Larson (Scott Pilgrim vs. the World, 21 Jump Street) for the role of Sarah Connor. Also Tom Hardy (Star Trek: Nemesis, RocknRolla) is reportedly being eyed to star in the new Terminator film as John Connor.
In addition, THR reports that this entry of The Terminator movie is being called "a time-spanning story that sees Schwarzenegger tasked to protect Sarah Connor as she grows up." The film is also billed as a Reboot, so most likely one can forget the previous movies and where they went or left off.
One could also speculate Schwarzenegger will being wearing the leather jacket and shades again as well, no shirtless or nude shots like in the previous films, otherwise someone might wonder why Skynet is building machine kill-bots that look like old dudes.
The actor who coined the phrase "I'll be back" in the first Terminator (1984), still seem to be words of truth today.
Details are still sketchy as to who will play the other roles or even the plot details, but the project has Annapurna Films allying with Paramount Pictures and Skydance Productions for this latest Terminator and they are targeting the release in theaters for the summer of 2015.