I think Duncan "should" re-sign with the Spurs once his current contract ends after next season. The Spurs will have the money to offer him the MAX amount, plus enough to offer another free agent huge cash.
By the looks of things, the Spurs could have a very young and talented group to surround Duncan with. Parker, Ginobili, Rose, Claxton, Luis Scola. Getting another big man to fill David Robinson's shoes via free agency should be their top priority.
Bottom line is, its all up to Tim Duncan. The Spurs should have the right pieces to surround Duncan with. I believe he should re-sign.