This is the most intriguing of questions. Martina is very devoted to Melanie, but I think she needs to at least augment Melanie's advise with the help of another coach, one who can help her serve. Joey and Martina seemed to get on very well, and he had nothing but praise for her work ethic and enthusiasm for the game when she played for the New York Sportimes. He would certainly be a candidate for the job.
Others who come to mind would be Nathalie Tauziat and Navratilova. Bad Gilbert is also available, but I don't think their personalities would mesh at all.(Brad strikes me as being about as much fun as a tax auditor.)
The other big question is, will Martina play doubles and if so, with whom. MY picks, assuming Anna does NOT come back, would be Kim or Gesila Dulko.