Since they are all adventurers familiar with the wilds, anything to do with non urban environments. Setting it in a genre to a popular film, is always a winner.
Some ideas
Warring tribes brought together for a common goal (latest Conan the barbarian movie, Ace Venturer 2)
Growing threat of a creature type (Game of Thrones white walkers)
Capture by bandits/slavers, work together to escape. Lost memories/amnesia (Saw I/II/III/IV etc)
Spies, sent to gather intelligence
Exploration (Kingmaker - Paizo pathfinder module)
Queer storm brewing, a strange omen. Eg blood moon/red sun which has allowed lycans/vamps to walk during the day.
Strange disappearances, discovery of a new tomb/treasure map. (Indiana Jones, Missing [Dungeon and Dragons Online - check the wiki])
Plague or drought, need to search for resources/cures.
Escort/merchant wagon (Jade reagent - Paizo pathfinder module)
I kind of having a bit of a mental problem combining the words "barbarian" and "rancher" in my head.
I dunno, I see that farmer as leading a horse to water and FORCING it to drink. ;)
Anyway, what about having the ranger hunting some predators that just happen to be near the witch's home, and having the barbarian be in the area because some of his herd went missing and he learned that there was a witch who might be able to aid him. In this way, the ranger and barbarian are already looking for the same thing (the predators) and the witch has an interest because her augury reveals that it might threaten her home as well.
-Forest Fire (source and nature up to you)
-Vengeful Forest Spirit
-Maurauding Beasts
-Logging Company/big business encroachment
-Flood
Hunter ranger could be tracking down someone who burned the witch's home to the ground, and in so doing, wanders onto the lands of the barbarian rancher's tribe.