I would expect some overhyped jitters in game 1, might break a back board if he tries a bank shot, but very little rust.
That said, it will be his 15th season and I'd still only expect another 16-8-3 type year in only 30-32 mpg.
On topic, I expect him to come in with the passion that he had in year 1 but the comfort level he had in year 2. I would not be surprised if he is an MVP candidate this year, which if he is fully healthy I think he might be. If his health is not right, though, I expect we'll know that soon enough as well and then his role will be subsumed even further into the team.
I quoted this post, though, because I think that even those of us that are huge fans of his tend to underrestimate him. "Only another 16-8-3", to me, completely misses the point. KG could average 16-8-3 for this team and be one of the top-5 players in the NBA. If he's healthy, I think he will be. I can't think of any other player in the league right now for whom the traditional box score stats are a less indicative measure of their production than KG.