According to TheExaminer action star Dwayne Johnson is looking to play a Terminator alongside Arnold Schwarzenegger in Terminator 5. It's an interesting bit of news since we don't even have a director at the moment. Besides, the recent talk that Nicolas Winding Refn (Bronson, Only God Forgives, Drive) met with producers. Dwayne is having a solid roll of roles lately and could make an excellent villain. They also include possible plot details that might make it a little more unique than the last two films.
UPDATE: According to a source this news isn't 100% so take as a massive rumor.
T5 may be a prequel set in the days of Sarah Connor's parents, marking the first time terminators have been sent back in time to kill a character that existed before both John and Sarah Connor.
Incredibly, Schwarzenegger may not play the role of a machine in the next film, but a potentially heroic human figure in or close to the Connor family. One theory is that Schwarzenegger's remarkable ability to protect the Connor family in the 1940s or 1950s with antiquated weaponry by today's standards ultimately inspired the machines to develop a terminator in his likeness that was eventually sent to 1984 when Sarah Connor was first targeted for termination.