I think a Carter comeback is sort of like a "Reagan third term"(Repubs around '87 talking about somehow amending the Constitution to allow Reagan to run again, because he was so popular): it's the kind of thing that sounds good as a fantasy, but would fall apart if actually tried.
Apart from the Republican wet dream, Reagan in '88 would have been noticably incapacitated on the campaigh trail, and would almost have certainly been trounced by any Democrat put forward. By the same token, as others have said here, Carter's home-building good-guy image would vanish overnight if he ran again, and be replaced by endless footage of rioting Iranians, Soviets in Afghanistan, etc.