Tsunade's roll as a leader is over. She was Sakura's Jiraiya. The loss of that person is what Kishi is using to show the progress of the characters. Shikimaru lost Asuma, Naruto lost Jiraiya, and Sasuke killed Orochimaru. She may not die, but she is definitely going to be at least left very weak. Bringing her back into the story and possibly even returning to the Hokage roll would completely disrupt the flow of the story. I doubt its something Kishi would do.