According to sci-fi blogger John Keegan, the "cliffhangers" at the series end were actually pretty well resolved;
As it stands, this could be the series finale. The writers left the
door open, especially in terms of Armageddon, but nearly every
character arc has come to a point of resolution. Walt’s legacy is
complete, Purdy has turned the corner on his search for redemption,
Bruce has found a life for himself, and Johnny is with Sarah and JJ.
There is one hint for the future, if the series is renewed for a
seventh season. Had Sarah remained with Stillson, her presence would
have prevented Armageddon (or so Herb’s visionary statement seemed to
In 2008 actor Anthony Michael Hall stated that he was hoping that there would be a sequel film but that the show itself "ran its course" suggesting that any further plot developments would have been written after confirmation was received that the show had been reprieved.
If you want an idea of how the show was likely to progress, you can view a show bible for series 4 here. It's pretty much "Monster of the week" territory from season 3 onwards...