First of all, this guy needs to come instead of signing with any team in Europe just like the last time.
I agree that he might bring about major improvement at the 5. The key will likely be how to let him play next to Duncan. I am no genius in basketball coaching. I could only recall the transition from Robinson to Duncan. No disrespect to Duncan (he's the main reason I became a Spurs fan), if Tiago comes, the era of Duncan playing in the low post with the other big man staying out of his way might end. I envision Duncan playing more in the high post like Mcdyess, so as to preserve his legs and concentrate on D.
Of course this is not an overnight process. Duncan's low post skills are still there. Tiago needs to prove he belongs to the NBA. As a fan I just keep dreaming on the best for my favorite team.