Hi!
Do you have any plot ideas yourself?
As I wrote before I do not intend to take part in a role play game at this time, but maybe I can give you some plot ideas. In the Great Valley forum I used some ideas from an LBT story I was planning. However, I have many LBT stories in mind and this one is probably the least likely to be written any time soon.
There may be a place in the Great Valley, where the river and maybe several brooks, run out of the Valley. If it is located in a depression it is likely to be flooded in case of a rain, and it may be that within few moments a muddy meadow can be changed to a torrential river pulling everything with it that was on the meadow. I named this place "The Wash" in the Great Valley boards RPG. One of the gang (Spike for example as we know he is not a good swimmer) may be dragged away by the floods, and unwilling to even think of the possibility that he may have drowned (and sure enough he didn't) the others set out into the mysterious Beyond to find him.
I could imagine the river running into a desert, a salt pan to be precise, one of the most unhospitable landscapes possible. It would be quite difficult to get back from there once you are in, with no means of orientation. Shifting dunes could make finding back even more complicate, and the run of a river (to meet the most striking counter argument against the whole idea) may vary as well as it's water level. The water of the river would most likely turn into an undrinkable, even dangerous salt water.
Scavenger characters could appear in such a horrible desert (which of course would result in many difficult questions to be dealt with). Maybe herds have to cross the desert time and again (how about Bron's herd? And why not combine it with a reappearing of Ali?). So long the story takes place within the desert however, it would mainly be a struggle for survival.