In all curiosity, could the Curator eventually become the Watcher that the Fourth meets at the end of his life? The Moment would've been prepared, after all.
That was the most different regen ever on the series--the most special. There was also a 4th Doctor episode where he aged a great deal--you might make something from that too.
Or maybe, like the image of Rose, this was The Moment....
It's the end... but the moment has been prepared for.