Seth Gabel is confirmed to be a regular next season.. we know that Red universe Lincoln is in love with FauxLivia, when we were introduced to Blue uni Lincoln, i believe there was an interview in which Seth postulated that Blue Lincoln would have been a part of the Fringe team if Peter hadn't been brought over, as it is he works for another div. of the FBI..
what if next season blue uni Lincoln is part of the Fringe team.. he replaces Peter (who didn't exist) as Olivia's partner & eventual lover over the last however many years.. they are a happy couple..
Fauxlivia, so resistant to crossing that line with Red Lincoln, gets to see what the relationship would be like if she said yes.. the same way Peter & later Olivia viewed the beauty of being together through the lens of his fauxlationship with fauxlivia.
Tension building & culminating with the bringing back of Peter, who Olivia is inexplicably connected/attracted to..
the result of this could be reminiscent of Roland & Jake in the Wastelands having both memories one of Jake's death and a second in which he didn't die... it could drive a person mad!