I know I won't get this, but I'd personally like to see all of the old cast come back for one epic season to pass the torch to new characters. More specifically, I think there is a storyline to be pursued with Buffy being much much older than she was when we last saw her. If vampires were the problems you have to overcome in your youth, why can't Joss & co. apply that same principle to the problems of aging?
The last time we saw Buffy in the TV series, she had shared her power with many girls throughout the world. The last frame suggests she is now free to live a normal life, but that also comes with a down side. What if not being the only chosen one, having to be ordinary-ish, and having to take a backseat to younger slayers gets a little dull after a while? What was once a burden may be welcome to a much more seasoned Buffy. I'd like to see her have to prove herself all over again and make her peace with all that she has accomplished, and what she still has left in her.
To that purpose, I'd like to wrap up any questions or longings fans had from the original series, and wrap it up in such a way that there can be no more repeats with the original cast. By the end of that first season reboot, it should be completely handed over to the next generation.
Of course, none of this will happen. It has been made pretty clear that if it does get made it will be with an entirely new cast and concept, but a girl can dream.