XBA said back on page one quite smartly, that the Rock is still young, and if he does come back for another storyline with great build up which he wants, and great biuld up would take a few months of promos. He could stay after and already become part of the roster for a few more years even.
If that does happen after the HBK or Cena or whatever match he returns for, I hope that he stays to help some stars become future heels/faces and he give them the rub into stardom. Stars like Morrison, Dibiase and Ziggler would be something that I would like to see interact with Rock backstage and in the ring, heck even Punk could get a rub, (their promos would be gold too).
Anyone remember in the HOF induction of the Rock's family that he said there were three people he always wanted to face, I haven't seen the tape since but I remember one of them was Rey Mysterio. That too is a possibility.