My guess is that either Ysayle, or her spirit, or Ysayle as Shiva, will play a major part for when we end up dealing with or saving Einstein.

If I remember the lecture correctly, primals don't die, but rather stay in Aether Land until they are summoned again. Assuming Shiva is similar, I really wouldn't be surprised if Ysayle/Shiva does make an appearance somewhere in the future.