Barb, the ending is another way Three Billboards reminded me of the Coen brothers as they are big fans of Unresolved Endings. With Billboards, there is, I would argue, a sense of resolution in terms of Mildred and Dixon becoming a ‘team’, so their respective arcs move them into alignment. However, the fact they are going to Idaho with the intention — or not! — to kill Crop Haired Man definitely leaves the story in limbo.
As you suggest, perhaps Three Billboards is a prequel to the Mildred and Dixon story in Idaho!