Sheldon and Amy's wedding is imminent; by the end of Season 9 they'll (surely) be engaged, and that's a whole stack of storylines right there. Sheldon's dream wedding versus Amy's. It'll be a spectacular event, and certainly one worthy of the grand finale. It would be the ultimate test of Sheldon's powers of compromise, a demonstration of how much he has developed as a human, and it would be a great chance to see every character one more time, whether they were classic one-off or a famous face. It could be a perfect ending to the show, though it may come too soon to be dragged on until the end of the tenth season or beyond.