I got a theory. Stick with me here.
In that last episode when the guy (Kevin, I think) with Mesa Verde was showing the Kim their expansion plans and the locations, one of the ones he mentioned was Nebraska. Jimmy/Saul eventually goes to Nebraska when he leaves Breaking Bad. Kevin seems like a big fan of Kim's work. With the expansion might want to bring her on board. Maybe she relocates, somewhere, and Jimmy doesn't go with her, hence how he ends up alone and full on Saul by the time BB timeline comes into play. But after breaking bad, Saul and Kim cross paths in Nebraska. She's obviously freaked out. He's got to explain this whole thing. They live happily ever after. Since no one ever does live happily ever after in BB/BCS to this point, It'd be a cool ending.