So, a few weeks ago, the producers of Fringe promised that one of the characters would die in the season finale (I assumed Astrid), but as we saw last night, two characters came to an end (sort of).
The Olivia of the future (15 years hence) was shot in the head by Walternate and died, complete with funeral pyre.
But then, Walter had a plan, to change history, so that it (along with the pending destruction of the universe) would not happen. Thus it was revealed that it was Walter himself who had originally built the ancient machine, and brought it back in time (along with the obscure instruction manual) so that it could be discovered in our time.
Only now, we learn that the machine had another function, that being to bring Peter’s consciousness to the future so that he could see the results of destroying the other universe and thus choose to stop it.
And so Peter did, by bridging the two universes so that both Walter and Walternate could work together on a solution. But this came at a price, as Peter blipped out of existence. The observers (who were observing from outside) said that the others had forgotten Peter, that now he had never existed (presumably, he caused some kind of time paradox that erased himself).
So, what’s up for next season? This article from Blastr talks about the Fringe producers hiring a new actor for the finale, who would be come a recurring character in season four:
Season four—and gods willing, there will be a season four—might bring us a new lady castmember. Producers are casting for a green FBI agent named Emily to come aboard for the finale and possibly recur next year.
As we saw in the episode, The new character was in fact a green Fringe Division Agent named Emma, who lives in the future. That suggests that we may in fact see more of the future world next season.