
Unknown, but probably something big and bad. Something so big and bad it can't be shown on the TV show. Of course, given what we've seen on the show, we can't really conceive of what that would be.
Sarah Michelle Gellar, Alyson Hannigan, James Marsters, Anthony Head (unconfirmed)
Joss Whedon
Unknown, but it doesn't look good
In the past, Joss Whedon has said don't count on this one until the show comes to an end. With the seventh season starting in a bit, and various cast members being tempted with potential big screen careers, the end concievabley isn't too far over the horizon.
But recently a new wrinkle has emerged. In interviews FOR SCOOBY DOO, BUFFY herself Sarah Michelle Gellar said she would not be up for a big screen version of the show once it ended. Reason being the original 1992 movie (on which the show is based) didn't do all that well, and Gellar doesn't think a version based on the show would do any better. Of course, Gellar also said she wouldn't be moving with the show from its original WB network to UPN, and we all know what happened there.
TBD