I'd like to see an end to the overall story, an end to the Man vs. Machines war that's been referred to all series long, and we finally see it in Terminator: Salvation. I'd like to see John Connor get some closure as far as his character, the terminators, and the war are concerned. There was closure when T2 wrapped up, but they came back with two more sequels and started everything up again.
I don't care if Arnold comes back, though it would be cool. But the story needs to come to an end.