It is quite clear how to work Arnold into future Terminator movies. His aging doesn't bode well with his terminator model, which would make no sense to utilize him as a terminator. Unless Skynet creates a new model with the body of a 65-year old man, which makes no sense. The way I see it, we have two options, one more entertaining than the other.
1 - Use age reducing technology. Similar to motion capture and what they did in TRON 2, Arnold can film scenes and the post-house can make him look 30 to 50 again. I suppose with technology anything can happen.
2 - Arnold comes back... as SERGEANT CANDY. Who is Sergeant Candy? Arnold filmed a scene for Terminator 3 that was deleted, but was an entirely new character. See for yourself here:
I propose that this clip be utilized for a future Terminator movie to then, introduce Sergeant Candy as a leader of the resistence. Think of it - Arnold AND John Connor fighting together against the machines (now, both as humans!).
Done and done.