I second the 'That's a shame' opine. Maybe have a two stage plan;
1) Have a Start, a Middle and an End. The Start (maybe three Acts to set the stage) and End (could be a single act) could be fixed, but the Middle would be more flexible.
2) In the Middle, tell stories until you have no more stories to tell, then tie up loose ends, tack on the End and go on to your next project.
It would make me sad to think this game will end just because you decided it would at the start, because there are so few really good games about (maybe 10 - 15), and this is telling a really interesting story. I think there could be quite a lot of interpersonal relationship expansion without interfering with the Main Story plot, and that should be considered.
Just one person's thoughts.