According to WWE Examiner, a version of the script is being developed specifically for Johnson, which would see him playing the unstoppable killing machine who is sent back in time to the 1950s to terminate Sarah Connor's parents. Arnie would portray a heroic human who protects the Connors using antiquated weaponry and ultimately inspires Skynet to create a machine in his likeness (only 30 years younger). If you ask me, that plot sounds a tad ridiculous - why not trace John Connor's family tree back a century or two, where the weapons would be even more antiquated and all but useless against a Terminator? Come to think of it, why not send another Terminator back to 1984 to assist the T-101? And if Sarah Connor's parents have encountered a Terminator before she was born, did they just completely fail to mention it to her? It must have already happened by the time of the first movie, otherwise Skynet couldn't have sent back a Terminator created in the image of the human who saved them... Ouch. My head hurts just thinking about it.
While it wouldn't come as much of a surprise to see Johnson joining the cast, the whole sequel-cum-prequel storyline contradicts Ahnuld's recent comments suggesting he'd be back in his signature role in T5, so I'd probably suggest taking them with a pinch of salt for the time being.