One of the things that's impressed me about Fringe is that it's managed to keep its long term plot reasonably coherent and deal with the problem of racheting tension by periodically re-inventing its background. The alternate dimensions/changing timelines setup means that they can do longish arcs, conclude them, and then start more or less fresh. I don't think the final season is quite as successful at this as previous ones, but they kept it up for a fairly long time.