Who is left alive that even knew Mace for him to come back and talk to?
Well, the two droids I suppose. It's possible Mace met Chewbacca during a visit to Kashyyyk before Episode III since he and Yoda are listed in written materials as perhaps the two most-traveled of all the Jedi of their era. And, of course, he can always carry on some kind of conversations with the ghosts of Yoda, Obi-Wan and Anakin. Even Qui-Gon, since we know he became a Force ghost like the others eventually did.