To be honest, my expectation for the ending will be one of them going back in time to stop Walter from getting Peter from the other Universe. Because if you get down to it, it all starts with that, considering it now seems as if Peter is the first Observer. So no Peter, no Observers. No Observers, no Observer who deflects Walternate, so Walternate actually comes up with the cure for Peter, Walter doesn't cross over to save Peter...a whole new Timeline starts.

It's sad for Peter and Olivia, but the best for the rest of the universe.