Mark Hamill, Harrison Ford, or Carrie Fisher. It would have been the biggest of all gimmicks, but you know the ratings would have shot through the roof and more than paid for the extra guest star salary.
No way could they have afforded Ford, but Hamill or Fisher . . . sure. Fisher guest-starred on ELLEN (the sitcom, not the talk show) and Hamill played guest villains on both THE FLASH and CHUCK.