A series of three "Fringe" books, written by Christa Faust, are expected to be released later in 2013. Each of the books will tell a past story of one of the main characters. The first book will be about Walter, the second about Olivia, and the third about Peter. They are each interesting ideas, but there are many other stories that can be told. "Fringe" contains multiple universes and multiple timelines, which results in an infinite number of possibilities that could be detailed in books, movies, and spin-offs.
To 2167, the invention of Observer tech
At the conclusion of the series, Walter had taken the Child Observer, Michael, to the year 2167, which is the year in which humans decide to sacrifice emotions for intellect and technology. Supposedly, Walter and Michael were able to convince those future humans that Michael's brand of "evolution" was better. Michael was an "anomaly" created by future Observers, exhibiting both deeper emotion than humans and stronger intellect than Observers.
So, how will they convince future humans that they are wrong to believe that intellect alone is more important? Will humans decide to create Michael's brand of Observer or abandon the idea altogether? If they decide to go forward and help humans "evolve," will Walter decide to take the plunge and use the technology? Will there be anomalies that turn out more like the old Observers? For all we know, Walter could decide that the rest of the team will be happier in the future and figure out a way to bring them to 2167.
To the other side
The old Observers were able to switch universes easily. Will the other side make the same choices as humans in this universe, thereby creating old Observers on the other side? If so, they could choose to take over the other universe, but at a later date than the events of the series finale, which occurred in 2036. They may even decide to take over this universe again, but sometime after 2015, when they took over the first time. We never got to see what happened after the timeline was reset, and Olivia and Peter were in the park with Etta in 2015. Walter and Michael could even return to help.
To 2020, an unforeseen complication requiring Walter
The Observers do not have to return in order for the team to be brought back together. Since Walter and Michael are in 2167 at the end of the series, they know what happens after 2015. Perhaps there will be some sort of natural disaster or a fringe event that kills one of the team members. Walter would most likely take the risk to become a living anomaly of time. Peter did it. Heck, Walter is already an anomaly. He shouldn't exist at all after 2015. Michael understands how these things work. Perhaps he will help Walter to break the laws of physics as we know them, once again.