Its obvious what's going to happen, they already set it up for us.
*Spoiler-ish*
Remember the one scene when Jigsaw has a flashback to his "pre-Saw" days? It has him in the park with his wife(?) and that's likley going to be the basis of the 4th installment. If they didn't plan to have one, they wouldn't have even included that tidbit.
I like the Saw series, and I completely agree with anyone that says its repetative, but I still enjoy it. The 2nd one was a little off, I think it was something to do with one of the writers of the original being absent, I dunno; but the 3rd one was alright. And frankly, as long as they continue to have that unpredictable (or at least interesting) trick ending, it'll still be an ok series in my book.