It's still premature to talk about Tim Duncan's heir but Al Horford is the best candidate.
No matter how much we resist the idea, Tim Duncan will be gone at some point and the Spurs will need someone to fill his role if they intend to remain contenders. There have been plenty of rumors about LaMarcus Aldridge being on the Spurs radar to be that guy. Yet as great as Aldridge is, it's not only hard to see him join the Spurs but he might also not be the right choice.
That's why the best case scenario is for Duncan to return for one more year so that a more suitable replacement becomes available.