It's true that the Maya/Josy story arch branches deeper into the story than probably any other plot in the game so far, but we were talking about curveballs.
As far as I know nobody saw it coming and just like Jade may have "ruined" many peoples savefiles for Sage, the scene when Josy suddenly stands in the door probably "ruined" it for all those who were just interessted in one of the two girls.
Now we have hindsight, we know how important the Josy/Maya relationship is to the story, but back then ? I mean it was the definition of a curveball and when it happend I was still sure that we could pick either one of them and not that it would end up being both or none.
It's entirely possible that DPC underestimated the reactions or flat out never saw them coming. I often wonder if he adapts on the fly when he notices that the players interpret things differently than he intended or if he just sticks to his script no matter what.
If I understand you right, than maybe curveball was just the wrong word. I think what you really ment is that his cliffhangers and teasers usually dont provide the same satisfaction as they hint at and sometimes even none at all(chad), which I, at least in part, blame on the long development time, but also agree with. Players have a looooong time to come up with what could happen and then it grows from there as other people add to it, creating a whole nother level of expectations.
