Arnold Schwarzenegger's red-faced temporary retreat from his post-Governator Hollywood comeback seems not to have affected plans to get a fifth Terminator up and running sooner rather than later. Last month saw Megan Ellison's Annapurna Productions grab a deal to make two more Terminators with new rights-holders Pacificor, and while the package that promised a returning Schwarzenegger and director Justin Lin is obviously now a man down, there are reports that Terminator 5** may have gained Paul Walker.
With Fast & Furious franchise director Lin at the helm, it makes sense that Walker might be in the production's sights, although it's not clear whether he'd be playing Kyle Reese or a new character. What's apparently on the table is a Star Trek-style reboot, with a torn timeline setting up a narrative thread to veer off from the established franchise without necessarily contradicting what's gone before. Reports that Lin would also look to bring in Vin Diesel and The Rock are entirely made up by us, and stem from a happy daydream we had just then.
As for Arnold's involvement, there's talk of his proposed screen-time being reduced to something more akin to a cameo, requiring less commitment from the big man, which may serve as an enticement: not so much a big comeback moment as a fan-pleasing toe in the water. Hell, he was in Terminator: Salvation and he didn't even need to show up.
But this is all hearsay and speculation at this stage, probably based on brainstorming sessions rather than concrete plans. Don't be surprised if this all turns out to be nonsense.