Yeah the Star Trek way could be possible and may work,Maybe some people from Salvation's future goes back in time to during or shortly after T2 and during the course of the film changes the timeline from T2's to a new trilogy!?
Since this trilogy would be half a sequel and half reboot then Arnold being part of it would fit like Leonard Nimoy in the new ST films,Since Arnold is very different now than in T2 or even T3 he should not play a terminator though,He should play a retired scientist or especially army general that became the template for the T-800's when he was younger